I’ve been there, but over the years I’ve gotten better at avoiding being in this situation.
If you are implementing something for yourself, and merging it back upstream is just a bonus, then by all means jump straight to implementing.
However, it’s emotionally draining to implement something and arrive at something you’re proud of only to have it ignored. So do that legwork upfront. File a feature request, open a discussion, join their dev chat - whatever it is, make sure what you want to do is valued and will be welcomed into the project before you start on it. They might even nudge you in a direction that you hadn’t considered before you started.
Be a responsible dev and communicate before you do the work.