Your SOs are flawed / poorly considered because it is entirely possible that you could have an election which unseated everyone.
If that happened you would then be disabled, by your own SOs from having a chair/mayor which you are required to have.
I can understand the possible thinking behind the previous decision - I wholly disagree with it, but I understand it.
It is a fallacy to assume that 18 months service on a T or P council is in any way a qualifying criteria for chair.
If you had the village idiot with 18 months service and a highly qualified, experienced and successful business leader or the like (pick your own example) your Standing Orders would REQUIRE you to elect the village idiot as Mayor.
It is self evidently idiotic and potentially unworkable but was “probably” initiated as a “blocker” to prevent a certain individual from standing.
Who says 18 months service will provide any decent experience anyway? It could just as easily be BAD habits that are picked up.
For my money - I’d be looking to remove that ridiculous requirement as no sensible and unworkable.