you know you can easily scale up and down instance sizes on aws, or move your instance to, say oracle. I've done both of these. The Ansible installer makes it very easy to move. you just down your containers and copy over the volumes directory for the move.
With these options you can start small and free. It may take a while to grow to 100 users. A 2 vcpu 2GB ram t4g.small on aws seems more than adequate and there's a free trial through the end of the year. It's arm64 though so small changes to the ansible lemmy.yml. The free tier t2.micro at 1 cpu 1 GB ram seemed too small.
Oracle has an always free arm64 image with 4 cpu and 24 GB ram plus a generous boot disk and 200GB block storage so you might as well start there. It will probably last quite a while. Maybe all the way through 100 users. The only thing is while I know it's very easy to change your instance type on AWS, I haven't looked into it on Oracle. But again moving your instance is very easy.