You adjust the relief via the truss rod.
You adjust the action by raising or lowering the bridges and by getting the nut at the right height.
The salient point is where & if the strings start hitting the frets when lowered.
If the strings are hitting the frets right down at the dusty end, there is probably too much relief & the bridges are too low in an attempt to compensate.
If the strings are hitting the frets near the nut - say 5th fret - then there's a backbow - to get over this you have to raise the bridges very high.
My fretless took a while to get right. The Bass Doc put a fretless fingerboard on my neck & it took a period of time for it to adapt to string tension & get the truss rod right.
There was a bit of choking around where the 9th fret would be. It took a few goes with the Allan key & about 8-9 days of settlement before I got the fingerboard/neck flat. I had to cut the nut down a bit & only then did I adjust the bridges to see how low I could get the action, now that the strings weren't choking on the fingerboard.
I now have a nice low-ish action which plays cleanly with my fingers & I can get mwah when I use my thumb. I could cut the nut down more yet.
I've found I don't like a *very* low action.
To me, when a neck is straight or *almost* straight (as recommended by Dan Erlwine in his book), there is a different feel - difficult to express, but the neck feels much more stable/stronger to me than with a noticable relief.
Get the neck almost straight (again, Erlewine quotes Fender that at the 17th fret there should be 3/32" ±1/64" string height above the fret), then get the nut height correct - again, Erlwine recommends that when the string is stopped down between the 3rd & 4th frets, there should be about .01" between the string & the 1st fret - roughly .02" ±.002 unstopped , then you can follow up by adjusting the bridge heights to taste.
Of course, all this depends on the fact that the frets are all level!!!! It's amazing the difference a decent fret job can make to getting a decent action. Also, the bottom frets wear with constant playing & the dusty frets stay the same, resulting in the bridges being progressively raised & the action getting higher.
G.