yes but tangled.org really does do most of that!
1. JJ as the VCS: tangled supports stacked PRs using jj change-ids. https://blog.tangled.org/stacking , we use it a lot to build tangled itself: https://tangled.org/tangled.org/core/pulls
2. Raspberry pi as a forge for a long time: also check, the git server shim is super lightweight, its just an XRPC layer over git repositories + an sqlite3 database. there are folks running it on a riscv board with 512 megs of RAM.
3. Actions are critical and they should be runnable on my local machine: IMO this ask is slightly misplaced. it is mostly your build-systems' job to be hermetic, run anywhere, handle cross-builds etc. it would be really cool to "promote" results of such builds to the forge itself.
If I could make my own GitHub
https://matduggan.com/if-i-could-make-my-own-github/