Like with all other online services, it's cheap until your collection grows. I basically outpaced Google Drive (at the time), and it would have been significantly more expensive to continue with their paid plans. So, I invested in hardware. LOL
But realistically, if you have a smaller photo/video collection, then I think it would be just as easy to spend $5-10 a month and forget about it. I was working with decades worth of images, so I would quickly outgrow any 1, 2, 3TB plans.
I don't mean to talk negatively at all, and their product looks amazing (especially with the added context you've provided).
For me, based on my own experience and the huge amount of storage needed to keep my photos/videos safe, it's not cost-effective for me personally. If the choice was any of the other paid services or Ente, I think Ente would be the clear winner from the sound of it.