It's rare that you'll actually have enough decent major family members to fill all the positions you want. If you want to keep them happy you'll have to accept that some of your positions are going to be filled with not-terribly-competent people - some of the less important offices, or maybe small governorships, usually do the trick. It's a very intentional balancing act between skill and connections.
Can only agree with that. For the "big offices", depending on government form and your current situation there are quite a few positions where a low attribute is generally or situationally acceptable and satisfying a family plus perhaps getting more PI for an extremly loyal office holder is the better deal.
Republics: High Priest and Physican (mattering only in the moment you sacrifice or start treatment; I know that the latter gives +0.0X health per month - but honestly...does it really matter?), Magistrate (when having low/no AE currently), Tribune of Soldiers (in peacetime). The Censor partly: Go for a high attribute, but OTOH even more important is supporting the right faction, IMO.
Monarchies: Here I think we have the most "expendable" offices...Physican (see above), Royal Tutor (see above for High Priest - unless you currently let him educate the royal offspring), Marshal (unless you are in war
and use mercs
), Master of the Guard (unless at war; I know about the impact on protection against assasinations - but I don't see them really often in IR. We just aren't in CK3-land here

) and Court Philosopher (if legitimacy is currently maxed out)
Tribes: Apothecary (...), War Chief (even in war times - recruitment speed is never an issue for me), Bodyguard (if manpower is maxed out; see also Master of the Guard above), Arbitrator (if low/no AE) - at the very beginning, even the Wiseman might count among them (Citizen happiness is important - but how many of them do you have at the start of the game as tribe?)
Governors are an option, if they only administrate little territory.
In peacetime, I often create or withdraw General jobs - by either concentrating forces in the hand of the ruling character or creating 1-cohort-armies for the sake of having a position more. 1-Ship-Admirals are a nice option, too - however, be aware that in republics those gain a lot of senatorial influence (so if you got that route, pay not only attention to the family, but also to the faction...)