It seems you're moving to a highload environment. On this road you should reconsider your tech stack choices. SQLite is deffinitely not an option. At least you will need Postgres.

Also decomposition to a set of microservices helps. E.G. your main current server doesn't need to run the same host or even environment that Post Finder. And on some point of time you will hit a wall by data volume and should decompose your data for hot and cold storage. For example Post Finder probably won't be used for old posts so we can move old texts to Sta HDD or even Hadoop storage.

You run a cool service, contact me if I could help - I will do it gladly. Here or at Telegram @br0nevik

Supported with full upvote.