Of course! Happy to help!
Yes, either Assistants API or the chat completions API. Either or should work.
Unfortunately, if by OpenAI UI you mean ChatGPT, then no, there is no way to integrate something you built with the API into ChatGPT unless you make your own “tool”, which would be your own API endpoint you built yourself. This is not recommended because, well, you pay per token with the API, but you do not make any money with a custom GPT (yet).
That would mean that yes, you would need to build your own UI.
It’s a particularly tough trade-off, I know. On one hand, you manually handle the knowledge files and deal with its quirks and limitations, On the other, you’re basically building it from scratch to incorporate the features you want in the way you’d want.
I would give it some thought to decide what would be worth the trade-off. Each person’s use-case is different.
I wish you the best of luck!