I don't intend to use them in series. (3 in series would be no problem for Pi anyway. If anything 3S might be advantageous because step down converters are not any less efficient than step up) The UPS hats I'm thinking of are for parallel and step up to 5V though. Example

I never heard of this even number limit. There are many 3p and 3s Li-Ion packs being used for Pi's. I just find their lack in a Pi hat form factor rather odd.

So I was going through my arcade wishlist on Amazon ( separate from my pi project wishlist ) and I came across this and I have a feeling it might come in handy. Diymore Double 18650 V8 Lithium Battery Shield Micro USB 5V/3A 3V/1A Power Bank Battery Charging Module (18650 2 Holders) https://www.amazon.com/dp/B07SZKNST4/