I would vouch for Thunder for Android and it has iOS version too. But I don’t think it can override the system font, so it is out of your requirement.

On android boost is best for me, proprietary but lightweight.

I can recommend Jerboa on Android. Used it since the start and never had problems with it (and its lightweight).

Voyager crashes for me on iOS too. Usually from scrolling too much in All. It’s gotten better but still happens too easily. Doesn’t happen as much in subscribed subs view or local.

I know Sync for Lemmy on Android does this.

Does not answer your question but I just checked and you cannot customize the font in Mlem