You can't go on like this! I'm in a similar, but not anywhere near as full on situation by any stretch. My partner works long hours and is often away, so drop offs and pick ups are on me, I work 4 days and barely see the children during the week due to wrap around, weeknight evenings are spent eating tea, doing jobs, packing bags then bed. Then I try to squeeze in any appointments, errands, quality time with DC2, housework, washing, meal planning, etc into the time between school runs on my "day off". It sounds like you are also the family social sec, taxi, PA to DH and the kids in terms of sorting out anything and everything they need. I also like to have family time on a weekend but we always end up just doing a day of chores one weekend day at least. And we have a cleaner!!
My DH chips in too, it sounds like yours is a lot more absent (even if it's due to work).
I'm also wondering if this is just life now - I've got my dream job so went up to 4 days for it and I don't want to give it up, but at the same time I feel like I'm just getting by week to week, I'm exhausted, and I can't keep up with it.
If it helps, I started by delegating anything and everything I could - to both husband and kids. Simple things like paying nursery, paying the milkman, paying the cleaner, ordering DDs repeat prescription - I just changed my contact details to DHs so one less mental load.
I started setting healthy boundaries at work and saying no to things if I needed to (not sure if this applies to you being self employed) but is there anything at all that doesn't necessarily need to be you?
I do all my shopping online. I try to order kids clothes online where I can too. I get the kids to set the table, tidy their toys, and they've even started wiping down the kitchen (eldest is reception age) every little helps 😂.
I am considering paying my cleaner extra to do some washing for me as well as cleaning. I've also thought about other paid extra help.
The only other thing I've thought is to reduce my hours at work. DH would struggle to reduce because of the nature of his job so it would be me. We can't do it all, something needs to give.
The other thing I've cut down on is stuff for the DC. We don't currently do any clubs, I don't do playdates in the week at the moment, and I'm selective about what parties I say yes to taking them to and I don't feel bad about saying no if it means I can cope better and be a better mum.
Is your DH work flexible in terms of could he drop a day for you to do an extra day so you're getting more of a balance?