There's no such thing as "fully mapped out". Just open StreetComplete. If there are no tasks left, go ahead and map all the individual trees, or benches, or yield/stop signs, or all the buildings so that they appear good-looking on a 3d render.
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I am confused as to the implication? Is it because you are dissapointed that you cannot contribute?
6 months ago, I moved to a town which was 8 years out of date.
It's been a fun time adding in all the nodes I would have liked to see as a newcomer.
What is the easiest way to contribute?
Install StreetComplete, open it, boom, a lot of easy things to do that will help some people out (especially with regards to opening hours, accessibility, addresses and such).
Once you want to do more advanced edits, install Every Door. It's not as obvious what to do there, but it still highlights issues that need to be fixed and is relatively easy to operate; however, you will need to start looking at OpenStreetMap wiki to take full advantage of it.
Then you can move to more advanced editors, such as Vespucci on your phone (wouldn't recommend doing everything from there), ID editor in your browser or JOSM as a proper app on your computer.
I will be honest, I stopped doing it because I got overloaded with all the different types of concrete slabs and stuff.

@pineapplelover If you find a quest repetitive you can disable it in the settings. @balsoft
StreetComplete has recently removed the concrete plates option due to potential confusion, for what it's worth.
@potentiallynotfelix guess you'll have to move again /s
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