I've numbered your statements/questions and will respond below:
No, it's a per month thing.
Agreed, and I'm working on alternatives.
Understood re: "serious collector stuff", but in general I'd advise that there are far better services for image storage than a forum and would in general encourage you to store important images on services designed for that purpose.
Image compressions is already in place.
I don't believe I can do so, no. I will however, at least look into it. Remind me later if it seems I've forgotten (I likely will).
Google sheets would be my suggested alternative.
I'll note that it looks like the primary issue is videos, not images. As I say, compression is already happening.
Consider all of this an ongoing and fluid thing. I'm, adjusting now to see what works. I've reached out to the service provider to see if my S3 configuration is set up properly; if I understand correctly all the images etc. SHOULD be going to an AWS S3 bucket, and thus not bump Invision to the next cost tier. However, the server space seems to be filling regardless, so I've reached out to them to confirm the situation.
If you want to commit funds to increase server space the best way would be through a monthly subscription. That is the most stable way for us to see what we can/can't afford to commit to.