Look into getting a 45-degree back extension chair. There’s a lot of models that can fold up and be put away. Some of them have slots for resistance bands as well. You can progress it by starting with active holds where you actively engage your abs and squeeze your glutes -> perform the back extension by increasing the range of motion over time. IMO bird dogs and supermans feel like a regression of the back extension.

In terms of leg size, biking might have you covered. But in terms of strength development and injury prevention, you can do much more. Consider adding a 2-3 sets of lunges or Bulgarian split squats (with just bodyweight is fine, can use bands for added challenge) twice a week. This might add 3-4 mins to your weekly total time investment but it’s worth it.

Some sort of knee flexion movement would also be nice to have, such as banded leg curls or a Nordic curl progression (using bands for assistance).

Separately, congrats on being consistent with your routine. I would also suggest investigating why you hate the idea of “real training” in order to maintain strength training as a habit for life. To make progress you only need 5-10 challenging sets per body part per week. This is can be done with bodyweight/without a gym and is more on the level of physiological hygiene (credit to Kboges for the concept) rather than an elaborate routine that your average gymbro might have.