@petey I’m not really sure what temps are the best, but I had heard low 80s would get them germinating fairly consistently. Since I was going to transfer them all once they had sprouted, I wanted to get as close to uniform germination as possible and it seemed quite successful at those temps.
I don’t think I’d start them in buckets if I was going to transfer them to tree pots soon after germinating, it just seems way overkill for how much they would grow and as you note it would be difficult to get them warmed through with that much height and soil. If you didn’t want to use flats, I’d probably just use whatever you are using for peppers and such. You could plant them now and leave them at room temp and then move them to the heating mats in March or so if you aren’t seeing them germinating yet. They will push root for a while before top growth, so small pots make it easier to dig around and see how they are doing. Once I saw many of mine with roots just starting to poke out, I left them alone and knew they would be pretty well germinated for when I wanted to plant them out.
There is another thread here all about starting pawpaws from seed you may want to look through for various experiences from different growers. I looked back at my posts and it seems the seeds were starting to germinate after about 2 weeks on the mats.
I started mine late spring comprared to most, since like you I was using the heating mats for other things like peppers, etc. and was busy with veggies, etc. so the heating mat gave them a jump start to get reasonable growth in the first year. To do it again, I’d probably put them on the mat in late march, although maybe earlier if I was going to grow in pots and could move them around easily. Since I was planting out in the air pruning bed, I didn’t want to have them sprouting above soil when there was still a risk of frost.