As it is, you only see new comments if you scroll past the post again (and your client has refreshed it) or if you open it directly. If your client hasn’t updated the comment count or if you refresh your feed and the post falls off, you’ll never see it anyway.
A “Watch” feature would solve this better. If you watch a post, you get aggregated notifications for edits and comments on the post. If you watch a comment, you get aggregated notifications for replies to it or any of its children.
By aggregated notifications, I mean that you’d get one notification that said “The post you watched has been edited; 5 new comments” rather than a notification for each new comment.
Then, in addition to exposing a “Watch” action on posts and comments, clients could also enable users to automatically hide posts that are watched, either by marking them as hidden or by hiding watched posts without updates.
If the latter approach were taken, notifications might not even be necessary - the post could just get added back into the user’s feed when changes were made. It would result in a similar experience to forums, where new activity in a topic would bump it to the front, but it would only impact the people who were watching it.
You can kinda get that behavior by sorting your feed by Active, but this could be used with other sorting methods.
Giphy has a documented API that you could use. There have been bulk downloaders, but I didn’t see any that had recent activity. However you still might be able to use one to model your own script after, like https://github.com/jcpsimmons/giphy-stacks
There were downloaders for Gfycat - gallery-dl supported it at one point - but it’s down now. However you might be able to find collections that other people downloaded and are now hosting. You could also use the Internet Archive - they have tools and APIs documented
There’s a Tenor mass downloader that uses the Tenor API and an API key that you provide.
Imgur has GIFs is supported by gallery-dl, so that’s an option.
Also, read over https://github.com/simon987/awesome-datahoarding - there may be something useful for you there.
In terms of hosting, it would depend on my user base and if I want users to be able to upload GIFs, too. If it was just my close friends, then Immich would probably be fine, but if we had people I didn’t know directly using it, I’d want a more refined solution.
There’s Gifable, which is pretty focused, but looks like it has a pretty small following. I haven’t used it myself to see how suitable it is. If you self-host it (or something else that uses S3), note that you can use MinIO or LocalStack for the S3 container rather than using AWS directly. I’m using MinIO as part of my stack now, though for a completely different app.
MediaCMS is another option. Less focused on GIFs but more actively developed, and intended to be used for this sort of purpose.