Oof yeah the Instantly domain situation is exactly why I always say - own your domains separately. Namecheap or Porkbun, never through the sending tool. Classic lesson.
Setup is pretty straightforward once you own them - connect to Google Workspace or M365, configure SPF DKIM DMARC, then plug into whatever sequencer you go with. Keeps everything portable.
On the Instantly domains - have you actually tried to transfer them? Most domains are transferable after 60 days, might just be inconvenient rather than actually locked.