OK, OP, I get it, to a certain extent. I'm very much the one who keeps my family afloat financially, and have a DH who tends towards the workaholic side of things. Being the one with all the financial pressures is stressful even if you're really enjoying the job.
First, you're in the absolute thick of it right now. Toddlers are exhausting. Nursery is financially ruinous. It does get a whole lot easier, even if that easier feels a long way off.
Second, you can't put yourself at risk of losing your job/burning out for your husband's unprofitable labour of love. There are a couple of paths you can go down here, some more extreme than others, but you need to set some clear boundaries for yourself and your family.
One way is to set out the boundaries you need to make it manageable. Eg DH and I always split drop offs and pick ups equally; and split mornings and afternoon pickups so that no one person always had the morning hassle.
So you decide what would make your work/life balance tolerable. What jobs does he need take on from you to make things more tolerable for you? Is it partly getting the kids ready in the mornings before he starts his own work? Is it taking responsibility for most of the admin task, with absolute clarity of who's doing what? Is it not working any weekends?
Another option - perhaps supplementary to the above - is to agree a timeframe for his business to become profitable after which he will agree to return to the workforce.
And nuclear is - he has to give up the more than full time work that is bringing in hardly anything and simultaneously preventing him from pulling his weight at home. It sounds like you'd be better off with him as a SAHD.
But ultimately he needs to be part of the solution, and he has to see your happiness and wellbeing his kids' financial security as more important than his business. How does he respond to discussions about this?