How do we allocate server resources for managing hundreds of websites and more to come?

I'm looking for some advice. We have a dedicated server for over 100 websites we manage along with emails hosted for each site. The server is setup with liquid web years ago. We have outgrown this server and are now at the limit edge.

With the server at its limit and now with new server technologies I'm wondering how to start allocating resources. All sites need emails as well. Some get a lot of traffic and others don't.

I'm thinking of 3 options:

1. Just get a second large dedicated server for new sites.
2. Start putting each website on its own digital ocean droplet? but then email? I guess we put email on its own cloud.

I'm juggling how to manage all the server resources as well as keeping in mind budget of course.

I would love to get some thoughts from fellow devs. What would you do? Thank you guys so much.