I think I've been where you are. We are still waiting assessment but we have tried lots of things and some have worked. Most importantly, ds knows we're trying to help him. I wouldn't try more than one at a time for a few days so you know if it helps, also you could revisit some after the crazy Christmas period even if they don't help now.
For meltdowns, some things to try - zero eye contact, very soft speaking/non verbal communication, hard squeeze/hair stroking helps some children to calm down but some need no touch. If you can spot the pre-meltdown and initiate these things all the better. We also find humour or talking about special interests helps ds regulate his emotion when getting near meltdowns.
For bedtime - ideas are visual timer on last activity before bed, same routine, weighted blanket, projector/night light, white noise.
For avoiding meltdowns, maybe look at PDA/low demand resources - what can you let go? We've let go of: getting dressed up to see relatives (sensory with clothing), sitting at the table when dinner is ready (hunger = too much pressure, we let him stand near the table and eat something then sit when he's ready), asking how he is verbally (we do thumbs up/thumbs down action to ask him), surprise presents (the anticipation is too much).
For you - gin/chocolate/tea. It's a journey to work out what might help your ds and you have got this.