>>I work alone, so I end up writing & playing all the bass tracks for my songs, but I'm not a bass player
I LOVE playing bass but I'm not sure that that makes one a bass player.
I had enormous fun making bass parts to Alistair's stuff - sometimes what he wanted and sometimes not. I loved getting involved in Donna's Xmas song a few years ago who was very clear about what didn't work (my first effort was MUCH to fussy - but that's how I played then) but - I think - appreciated the bits I did that she, as a bass player too, wouldn't have done that way.
I've watched a load of Steely Dan videos over Xmas and wow! so so good. So practiced so tight so clever but with a lot of room within that. If you have access to the 'making of AJA' classic album DVD it's fascinating. Fagen and Becker were VERY sure what they wanted and wrote a lot of parts; AND Walter Becker was a bass player so that must have made it even harder. There's a bit though where Chuck Rainey goes - 'I know what they wanted but I know what would work. So I did it anyway. Just cheated a bit to get it through.'
The drummers and bassists in the various Steely Dan performing bands are astonishing.
Listen to the bass lines and learn so much for each is a consummate pro. Sometimes they play a root note first time and when you expect they'll do the same again they do - and sometimes they don't. So much going on that is probably unnoticed but without which it would be pedestrian and less good.
So much bass playing as I see it is a balance between several things - being solid; playing with the drummer; playing syncopated with the drummer; playing with the lead player as an equal; many others especially just 'being right' whatever that might be.
I played with a fiddle player a while back who opined that he thought I was just a bassist who was really a repressed lead guitarist. I dare say that because my bass heroes were in no particular order and missing out many Jack Bruce, Jaco Pastorius, Ray Brown, Stanley Clarke, Stu Hamm etc Each in their own way is quite up front in what they play and has a part in the whole. Jaco through his sound and style is perhaps the most recognisable? They are soloists secondary to their role in being a part of the music they are a part of. From memory (though I'm just listening to it now) Cotton Avenue on Joni Mitchell's Don Juan's Reckless Daughter is an interesting mix of Jaco playing in all sorts of ways. After his solo bits at the beginning and those noises, when he comes in in the main bit he is solid as a rock but hell makes it move and still has time to growl and play harmonics etc And leaves lots of space when it's right. And he doesn't bore. (Haven't heard this for a long time and gosh!!)
You have so many choices as a bassist to tweak harmony and rhythm if you are so allowed and if it's appropriate.
It's why people who don't play bass rarely have the lines they've written played.