Excuse my ideas but I don’t have and FP4. On my FP3 I can turn either SIM off, on the occasion I use two. The option comes under Networks and Internet > SIM cards
I am also looking for a way to turn off my business sim easily, ideally automatically at certain hours or days.
If I turn it off manually trough the settings and back on when I need it, the previous settings as to which sim I want to be used for which service get lost.
I totally agree with @jeanjonas I recently changed my phone from Samsung S9 to FP4, in S9 I could have different signals depending on which simcard the call came in from. I use both sim cards in FP4 (one is e-sim) all the time in order not to need two phones with me. Different ring tones for each sim must be addressed and cannot be a big issue to solve in terms of software coding.