My uptime monitoring business made a similar migration (AWS Lambda to fly.io), and I ended up rolling it back a few months later.

I wrote more about the move to fly.io here: https://onlineornot.com/on-moving-million-uptime-checks-onto...

and (part of) the move back to AWS here: https://onlineornot.com/scaling-aws-lambda-postgres-to-thous...

Edit: forgot that second link doesn't actually explain that I moved off fly.io, will write a follow-up.