So, I installed a new passenger side motor mount, and the startup thump is 50% better, but still there. But the mount was completely shot after 4/5 years (mine's a 2015) -I could move the old one around 1" in any direction. It feels better, and has made shifting gears a bit better too. I guess I should consider the driver's side gearbox mount as the source of the start up thump, but it's not so bad now that it worries me, so let ir be..
A note on installation is that when I jacked up the engine under the oil pan to just support it, the old mount had sagged so much, I had to jack the motor up at least 1" from just supporting it to be able to remove the old mount