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Vanneau's suggestion about glue exercises like (single leg) glute bridge as well as some other core exercises like plank and a little yoga has so far made a huge difference to lower back pain off the bike. Stretching on its own did almost nothing or only temporarily alleviated pain. Some tests also indicate that I don't really have super-tight hamstrings (doing a yoga forward lean after a round of greeting the sun I can pretty much fold in half) On the bike I am getting an inline seatpost to move the seat a little further forward after sliding around on my saddle to feel out different positions. Too far forward might not recruit the glutes enough (as someone posted a while ago), and it feels like too far back may over-recruit some of the muscles around the hip.
Sveinung-
I think gluteal engagement partly depends on pedaling "style" and also how much forward rotation you have in your pelvis. Therefore, if you're going for an inline seatpost, you may find you need to lower your bars further which'll mean you have to rotate more and hence engage your glutes.
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It's an odd one. I always assumed I had tight hamstrings; they felt tight. But the bike fit showed I didn't.
I'm hoping that working on glute strength will gradually eliminate my excessive lower back movement and remaining on-bike issues.
macaroon
Vanneau
This sounds like exactly what I have, albeit maybe slightly higher up the same group of muscles. They start feeling stiff, contracted and painful over longer distances.
My bike fitter pointed out excessive movement in my lower back during the pedal stroke as the key and suggested that the glutes were not doing their job of stabilising my back. This caused the back muscles themselves to try and take up the slack, hence the pain, stiffness etc.
The solution was stretching and glute exercises (single leg glute bridge, clamshell etc) more than bike adjustment. There does seem to be some improvement if I do these both before and after time on the bike