How do you refuel?

I break things down into small wins whenever I'm tackling a long-term project (either for my full-time job or for my product), but I find that I have a greater need to break things down into smaller wins to keep myself productive during those last few hours of the day. Every small win almost acts like fuel for me, and breaking them down even more seems to be helpful.

Great question, I would love to have the formula to recover as well.
I haven't found one yet. However, as soon as you have some momentum you need to keep it going. With momentum I mean, the urge to work on the project. Not meaning traction here. This comes later and will give you a lot of reasons to work on your project anyways.

While writing this another I remember to be full of energy about my side hustle when I was able to talk with others in person about their projects and share mine.

If possible, look for another job that starts later.

Then you can have your best 4-5 hours channeled towards your startup.

And the worst of you reserved for your employer.

For me, it's much more refueling to find something that makes me present moment vs clearing my mind.

When I am present-moment with something, my mind gets off work/projects completely. I've tried a lot of things but some of them just give me time to relax and think about my business (which doesn't help to refuel).

For me, running is more mind-clearing because I just wind up remembering that thing I forgot to do or that idea that I wanted to try that will grow the business.

I go to a local gym with classes (crossfit) and workout with other people for an hour. We spend a little time talking before a grueling workout. It's pretty rare my mind drifts off to work at this time.

Reading fiction kind of works but it sometimes feels like learning/training instead of fun.

Playing video games and watching NBA makes me present-moment as well. Need to find a replacement for these one to reduce screentime.

Playing music is a good one for some people (wish my wife would allow a drum set :) ).

Keep experimenting and you will know it when you find it.

I have also found boredom really gets me ready for work.

I'd remove the running, honestly.

Focus your remaining time and energy on your startup; they're way too precious.

Also, try working partly in the morning as well; chop up your startup time.
Do the things that are most focus-intensive in the morning, then the routine/tedious in the evening.

I work out -> eat -> meditate -> play guitar (30 mins) as soon as I get home and that usually puts me in a great spot to start working again!