SAH-parenthood is a real exercise of love. It sounds like this certainly isn't in short supply for you.
I find a great way to engage myself while still being "involved" in play is to wake up a few minutes early and scan pubmed, the markets, the news, business cases, or some other resource to look for novel business or scientific ideas. Armed with some fodder, I can brainstorm for business plans, etc. during activities where my son takes the lead.
It sounds trivial, I know, but it's a small step that has restored considerable personal fulfillment to my harder days. You mention your business as a source of fulfillment, so maybe this might offer a helpful outlet?
