Honestly the only thing that actually helped to a sustainable degree was medication for DS (age 7). I was already on it and it wasn't enough to help me. It helped me a bit, in that it massively raises my threshold to explode/overwhelm, it helps me actually think ahead because being 3-4 steps ahead of them is absolutely crucial, although I still find this exhausting, and it helps me sustain attention when playing with them, which always seems to "fill their tanks" (naff expression but it does seem to work a bit). It also helps prevent me from being an absolute hot mess which IME is incredibly unsettling for ND kids. I also have a DS1 who got the worst of our combined unmanaged ADHD when he was little and I couldn't even see how the chaos affected him because it was the water we were swimming in. I did not have that chaos with DS2, mainly because I was diagnosed, medicated and doing better, and massively because of DH's support too. However I was still drowning before he was medicated.
Some things that did help pre medication, even if not enough.
DH being heavily involved and giving me actual specific times off, including overnights. Me having an evening hobby which is outside of the house and NOTHING to do with parenting (choir - very regulating :D)
Big Baffling Behaviour book by Robyn Gobbel. Understanding the concept of dysregulation/stress behaviour in general (other good resources: Mona Delahooke, Conscious Discipline). IME most of the hyper/agitated behaviour is dysregulation related.
Having a set response to go to when DC dysregulated. The book above helped, and using the body language cues to communicate I am not a threat. But also just really understanding how to de-escalate and disengage. You must not engage with dysregulated behaviour as though it is literal. I find it almost impossible not to, because I take everything literally, I find it really hard not to answer a direct or indirect question, and I want to answer the insane requests or accusations. But everything calmed down a lot when I recognised this is dysregulation talking and not actually a request. ADHD Dude is good here, even though I don't agree with everything he says.
Figuring out what makes your child feel safe/unsafe and hence what is triggering the dysregulation. For example for DS, one of his big things that it took ages to figure out is that it makes him feel safe if he knows what to expect and how to predict what will happen. This is common with autism, although he is not diagnosed, only with ADHD. But when he is going into a new situation/environment, or when something in his familiar routine changes, it unsettles him massively. So things like social stories, visual timetables, now/next boards, telling him in advance what to expect help him a lot.
Some common areas for ND kids which can be involved in a feeling of safety (and most of them find more than one apply) are sensory input, whether demands/expectations are being made of them which they can manage or which are too much, proximity to a regulated, safe/known adult (so they can co-regulate), the general regulation level of others around them, time with a special interest, autonomy/control, routine - you might be aware of others. If you know what helps him feel safe, or what makes him feel unsafe, then sometimes it's possible to increase the safe thing as scaffolding, or reduce/avoid/shield him from some of the "unsafe" thing, which can reduce stress level across the board and help reduce dysregulated behaviour and/or make it easier for him to cope with other sources of stress e.g. school/sibling stuff.
Understanding own regulation, esp sensory stuff, and capacity to handle stress and trying to account for this in advance. I did not realise until DS was medicated (so is now unmedicated approx ~2 hrs a day) just how much capacity I need to spend time in his company unmedicated. He is an absolutely brilliant kid but he is so incredibly intense and it is a lot. Unfortunately DS1 was very similar, he is much older now, big age gap, but I didn't know half this stuff when he was little. He did calm down a lot by age 7, and then again during the first year of secondary, but I was so burnt out from trying to manage everything our relationship really suffered even so. We have reconnected now he is a teenager but I do honestly feel terrible about those years from 3-7 where I struggled to cope and then the few years after that too.
Also this is a weird one but unlearning a lot of things touted by the internet e.g. some "gentle parenting" doesn't work and just increases dysregulation because it's so unclear. DS does massively better with a very minor, temporary, unrelated sanction for specific, predetermined behaviours given consistently than he does with a million talks, connection, engaging with his feelings etc or me giving him endless chances, benefit of the doubt etc and then one of us getting pushed past our limit and then giving a related consequence in the moment because this seems utterly random to him. He also does much better with rigid rules rather than flexible ones. But OTOH it does not work at all to try and "show him who's boss" and I don't get into power struggles because it ends badly. I tell him I am stepping away to take a break because I am angry, or that we can talk when he is calmer, and I put a locked door between us if necessary (bring youngest with me) or sometimes I recognise that me wanting to get into a power struggle means we all need a reset and we either go outside or I just put the TV on. Which I know some people will be horrified by, but it is a survival technique really at times. Instead of "parents in charge" we just have house rules that everyone is supposed to stick to, although it is the parents' job to look after the children which includes sometimes preventing them from doing something that would break a rule. DH is a bit better at having inherent authority, I do better when the rule applies to everyone, and it sounds like the sanction just magically happens and I'm powerless to prevent it. He's not stupid and I'm sure he knows I'm the one deciding/enforcing it, but it helps a lot anyway.
For the sibling dynamics esp when they were 5 and 2 (I have three DC) I basically resigned myself to being sort of a combined bouncer and translator of intent and body language at all times. So I would literally sit next to DS2 and coach him through what I could see DS3 doing, what I thought it meant about his intentions, keep my voice very low and calm, explain somehow quickly in sentences DS2 could follow that DS3 was just looking and was not destroying and we could perhaps give him his own bricks, or OK you don't want him to touch that part, DS3, look, come over here build here instead. Often physically blocking. Janet Lansbury's "I won't let you" and "Sportscasting" was very helpful. Or modelling how I wanted him to talk to DS3, so he could try it out. And we set up a lot of separate play spaces which was often things like we had two rugs, so they would have one rug each and that was a clear border, and I encouraged the idea that sometimes you need space and that's really OK. I was very conscious never to make it a threat/punishment that I would have to separate them, I presented it as meeting a need, like you're hungry? OK let's have a snack. You need a wee? Let's take a toilet break. You need space? Let's make some separate play spaces. Totally neutral/positive meeting his needs. They get it now and will actively choose to take space if they need to and they have a really positive relationship.