Does this ID change if you provision a new eSIM? Would it be worthwhile to activate an eSIM on my phone and write down this ICCID for safe keeping in the future? Is there any other way to get this ID without having an active eSIM?

There is an ICCID, that is usually burned into the eSIM. However, SIM and eSIM are actually a tiny embedded system with a rudimentary Java byte code interpreter and you can insert firmware to dynamically swap between the reported ICCID. Usually this is a trick employed by MVNOs to swap between profiles based on the available networks to select the MNO that the MVNO has a more favorable contract with.