How FICO scores different kinds of installment loans is conjectural -- nobody knows for certain.
In your case the SL payoffs will likely NOT hurt your score since (as you observe) it won't significantly change the total percentage of how much you owe on your open installment debt -- but that is only because you have these gigantic mortgage loans.
But here's one way to avoid trying to second guess how the scoring algorithm works. Reach out to the company that is handling the SLs. You should find out if you can make big payments that will cause the next payment "due date" to get projected far into the future. Many companies permit this. If so, you can pay the principal way down as soon as is convenient, which will cause the interest your loan generates to become neglibible. But at the same time you can keep the loan(s) open.
One nice benefit of that is that, when you do pay them off finally, they will be that much older, and therefore they will be helping out your AAoA (Average Age of Accounts) for that much longer.
Incidentally there will not be any immediate score damage to your AAoA if you do pay them off early. The accounts will continue to age even when closed. But eventually they fall off your report and THEN you will lose their benefit to your AAoA.
Bottom line is you should do whatever you want regarding the SLs, though keeping them open but almost paid off has a marginal advantage IMO.
