I'd recommend iodéOS instead of /e/OS as the former is considered a bit more secure than the latter, see overview of Custom ROMs here (summarized from a well known German IT-Security Blog). You can also buy a FP5 with iodé preinstalled.
No app is completely uninstallable. But the one app in Iodé that can't at least be uninstalled via system settings is the Iodé tracking protection.
Iodé is considered one of the more secure custom ROMs and friends of mine and me are quite happy with it (running on a Terracube 2e, a Fairphone 3, and a Fairphone 4).
Might be worth taking a look at David Zindell's "The Ea Cycle".