Great advice here, I have been playing bass for almost 38 years now and I still have things to learn every day.
My growing edges are around sight-reading complex rhythms, and the finer points of playing fast while staying relaxed and grooving.
...And not getting distracted by gear...
I will second the "make time first thing in the morning so no matter what you get your time in for the day"
Also, 5 or 10 minutes of playing with truly focused attention is more productive than an hour of distracted noodling, though drilling technical stuff while watching TV or whatever can help to build muscle memory.
And as @kmon says it's very possible to develop your musicianship without a bass, especially by doing ear training to recognize intervals and chords. Also some dear friends lived right next door to "Senator" Eugene Wright (RIP!) - the bassist from the classic Dave Brubeck "Time Out" era quartet - he said when a bass wasn't around he'd practice fingerings, vibrato, etc. by wrapping his left hand around his right forearm as if it was the upright neck.