Ext SSD booting

I have a 2013 iMac 27 inch that I want to keep exactly as it is (Running an old version of logic pro utilising a second 2010 iMac 27 inch in target disk mode). However, I now need to be able to run a new operating system on it from time to time without disturbing everything that is in place on the internal SATA Fusion Drive.

Is it possible to turn off the iMac whilst booted in Catalina, plug-in the SSD either to the thunderbolt 1 or USB2 ports, and start up again choosing to boot from the external drive and CRUCIALLY, then go back to booting from Catalina later on? I understand a bit about root patches and especially modifications to graphics cards, does anybody know for sure whether any changes will impact it starting in Catalina (once it has been booted in Sequoia?)

Also, will the 2010 iMac still work in target disk mode when the 2013 iMac is booted into Sequoia?

I strongly suspect the answer is no, but the other option I have is to use screen sharing from a MacBook that has OCLP sequoia running on it, so that way I have sequoia running on the larger more visual screen and possibly even the 2010 in target desk mode at the same time ? I could try that for myself. Part of the need for this ridiculous amount of screen real estate is with logic pro it’s nice to have the mixing desk on one monitor and effects and tape transport on the other. When it comes to running Microsoft Office for work situations I could suffer the laptop as the teams head unit. Working for several different clients throughout today often means hundreds of windows open, but I can live with that I guess if booting from an external drive will screw with my logic pro.

Thanks in advance