Premature optimization is the root of all evil :).
It sounds like you are finding ways to tie into the services you need, and that Anvil is providing value in being that “nexus” - no small part because it runs Python on the server and anvil.js provides that front-end escape hatch (can you use wix from Anvil.js?). Adding microservices to your Anvil app is totally reasonable in my opinion, where Anvil really is just orchestrating and making API calls or whatever. Same goes with ML/AI etc.
Don’t underestimate your scaling options with Anvil either - sounds like you are on a Personal plan, so you can grow into Business or Dedicated as/when the time comes. Suffice to say, it sounds like you are very far from any “limits” you might face on the Anvil side!
Finally, if you do move to “something else”, the problem you state isn’t solved. If you want to work with MongoDB, integrating ML/AI pipelines, with some AWS services for video processing etc, you’re STILL going to need something to hold it all together.
Aside: You CAN upload very large files in Anvil. See this post by @owen.campbell: