I have a ton of custom filters myself, so I've got a couple recommendations.

Firstly, just try filtering by sender. If you've got an account on a given forum, shopping site, etc, then make a filter that automatically applies a label for Forums/Shopping/etc based on the email address/domain of the sender. If you know that lemmy is going to send you specifically forum related updates, set it to automatically label any email from your lemmy server as Forum. You get the idea.

If you want to quickly make a filter for a given sender, select the email, hit the Move or Label button, then before clicking Apply, check the "Always move/label sender's emails" box. This creates the rule for you. It just depends on if you want these emails moved to folders, or just tagged with labels.

Secondly, use keywords. So many sites (especially shopping related ones) are gonna be using the same terms very frequently. Think "% off" "black friday" "deal" "sale" "x months free" etc. You can combine all of those into a single filter with the ANY condition set. (This is what I do to delete all the promotional mail from email lists I'm on that are only part promotion, part content.)