I'll try a make menuconfig then, see what that looks like.
Applying your patches (all of them in that PR), is that a matter of cherry-picking them one by one with git? I don't know how to apply the huge patch that hnyman linked.
I've learned my tricks from this post after cloning a repository/branch:
./scripts/feeds update -a && ./scripts/feeds install -a && cp diffconfig .config && make defconfig && ./scripts/getver.sh
make -j5
--EDIT--
I've started make menuconfig and it's a bit overwhelming. I see a lot of stuff that I think I should select/enable, but I don't see "experimental kernel build" somewhere. If someone can give the the line I need to add to my diffconfig so 5.15 is built, I'm happy to build and test.