In the days before covid-19 I had to come up with basslines for several original songs guided largely by being told a 'feel' and having the changes shouted out the first time round as they were bashed out on an acoustic.
As the writer was a typical singer/songwriter/busker the more vigorous songs had the embryo of a bassline in them already, especially where he was putting in bass runs to lead between chords. Others had a distinctive rhythm that suggested a riff.
For the trickier ones a strategy is to play arpeggios over the chords, and as you get comfortable with the pattern start improvising over the changes and a walking line will emerge.
The other thing no-one seems to have mentioned is - is there an existing melody? Does it have sung words? If so, they will define the key better than any amount of theorising!