Kind of comment only (just to encourage further your thoughts direction and in order to learn something from you): As original Librem 5 charger uses PD3.0 protocol (and guessing somewhat that the PD2.0 negotiation is out of question here as well) I do not see that any of docking stations mentioned within your post uses (is based on) pass-through PD charging (but didn’t researched controllers of those hubs at all, certainly not extensively), as input and in particular (if matters here?), as PD3.0 power output standard (directly from used hub) when connected to the Librem 5 USB-C port. In your shoes (as I don’t have L5) I’d test, in order to expect that things might work the way you want (to have it for your purpose) those two devices: C31NANODOCKLANPD (2109:0102, see second link/list) and ZDSHB01 (2109:0103, also second link). Also, you might ask the author of linked thread if this i-tec docking station usable under PD3.0 (only), without battery of course.
Thanks a lot, great post anyway!
P.S. I saw another i-tec docking station that might be tempting if above one works as expected. But anyway, ZDSHB01 is already advanced one as USB-C(1) provides with 30W just enough power for every Librem 5 feature (as said, all here is just current thought of mine, nothing to add/confirm).