Some Sensible Alternative to Openrouter?

Hi. I'd like to try some LLM provider on the likes of OpenRouter. I'd use it mostly for teaching (physics): double check assignements I give to students for consistency, clarifications on how to present concepts and so on... Most likely, I _wouldn't_ use it for coding.

I'm not really familiar with the usage of API, so I'd need the service to be readily usable i.e., to provide an internal chatbot and, obviously, to be reliable. I find OpenRouter price a bit confusing (maybe it's just me not being familiar with it), apart from some odd fees they charge when you top up? This is somewhat important because my use wouldn't be so intensive, so I'd prefer to buy a limited amount of credits each time.

Soma part from OpenRouter, what are you people using and what's your opinion of it?

Thanks a lot in advance!