As someone who is still hostimg a forum, I would not suggest this. I have also tried to reuce footprint of the forum using external hodting services and this has always ended up badly.
This is one of the reasons why I'm not tempted to open up a lemmy server, even though my hosting plan allows 3 subdomains. Used space will rise quickly, even if it's just images... if videos are allowed as well, than all hell will break loose, even if they are prodessed and reduced in quality and resolution server side. I've seen it happen before, it's a nightmare to revert things afterwards (users complain), not to mention you can't rever the damage, that space is gone and that's that.
My estimate is that, if only images are allowed and images are processed in webp, it'll take about a week for a quite busy instance to reach the 1GB mark... probably a lit faster if it's an NSFW instance (probably a few days).