This is pretty old thread but yeah I ended up spinning up my own Postgres instance so I could use dedicated processors.
I haven’t tested this quite yet but you can probably do this now with this new update to Postgres: Improved Postgres Clustering with repmgr - Preview - #2 by shaun
Fly has been working on a new system for Postgres called Flex that is based on Machines API. My guess is that this supports everything that Machines API does.