I think by official OP meant stuff like Infinity is a FOSS client and not the official reddit app instead of alternatives.

I believe Pixiv is exactly what you describe for artists, but not open source.

You can refer here and check if any meet your needs. Some of the top ones are obviously not the ones you require so do scroll and try out the others.

https://alternativeto.net/software/imgur/?license=opensource

I didn't get why RSS would be expensive. Are you talking about time expensive?

I personally found Nougat to be the best, and then they focuses more on UI than features. (They also stopped the naming after that)

Huh. Feedly and Inoreader have decent free plans though.

(They aren't open source though)