@@Jarmfam, welcome to the forums!
General rule: Let the Boy Scout Handbook and the Scoutmaster's Handbook be your guide. If those documents don't give a restriction, it means troops have latitude.
You most definitely want your SPL to be a lad the other PL's trust and someone who wants to continue to develop his leadership and management skills. So, yes, if an incumbent wants to ask the troop to allow him to retain his office, encourage him to do so. A boy on his second term (however long a term is in your troop) can learn more about tour planning, vision casting, and training his replacement(s).
From the school of hard knocks:
Never dole out leadership positions based on need for rank advancement.
Ban adults from bribing boys to projects by saying "It counts for service hours."
Merit badge classes during troop meetings: no. Meet the counselor evenings: yes.
It's not a problem if it takes five years to become a first class scout.
The advancement method helps with skills acquisition only. For skills retention one needs the patrol and outdoor method.
Patrols need physical distance (up to 300' if you have an open field, their own rooms if your chartered organization is that generous).