Filler was only a thing in long running shounen shows. Have you ever heard of a romance show with filler? Cause they don't exist. They were always 1-2 cour shows on tighter budgets and most of them didn't even get to adapt 50% of the manga/LN let alone the ending. The last complete adaptations were Saekano and Oregairu and both had to cut a lot of content to get there. You should give up on romance anime unless they are originals.