Hi @dealing_with_doors welcome in the Zynthan’s world, where adventurous folks explore music, electronic and Linux based software.
The new ‘Raspberry Pi Touch Display 2‘ is quiet … new ! And, I believe that you could be the very first one here with that display under hands.
So, for sure, as is, there’s no “out of the box” working config in the webconf display configuration tool and I hope you are prepared to be an explorator too 
Things are moving very fast on the RaspberryPiOs side (wich is the base system of Zynthian). I would recommend you to,
- first, do all your test on an up to date RaspberryPiOs system and see if you can get the landscape view working correctly. Be aware that Zynthian use the old X11 display server. Now, the global (not only RaspiOs) way to go is to switch to Wayland. It’s been a while I didn’t play with RaspiOs, but (I’m not 100% sure about that) I’ve read elsewhere you can select wich display driver you want to use on RaspiOs.
- then, if this was successful, check /boot/fimware/cmdline.txt and /boot/fimware/config.txt files and compare this setup with what you’ve already have on your Zynthian box (also compare the kernel versions, I’m not sure Zynthian’s one is aligned with the latest RaspiOs)
- adapt these files on your Zynthian SD card (when tinkering with RaspberryPi and Zynthian, it’s a good idea to have a couple of SD cards. eg: one with latest RaspiOs for hardware dysfunction solving, one with your current working setup, maybe one if you want to test all the new features in the Zynthian testing branch called “Vangelis”
Regarding point 1. after a quick search, I’ve found these topics:
https://forums.raspberrypi.com/viewtopic.php?t=379738
https://forums.raspberrypi.com/viewtopic.php?p=2280504
where there is unfortunately no [solved] tag
I hope you will get the display to work correctly, so that you can after that scratch your head again with touchscreen functionality !
Good luck, and do not hesitate to give feedback and ask questions.
EDIT: it looks like the overlay you use is the wrong one:
dtoverlay=vc4-kms-dsi-ili9881-7inch