Oh beautiful Jen!
I think thinner can be fine for rucking, dpeneding on size of baby and blend. Nebbia, for eg, is quite a thin linen blend and it's great as a ruck and in a double hammock. (I reckon, anyway)
But, if you're looking at something that will last longer as a rucker and straight cotton, then yeah, you would probably want something a bit heftier and grippy is good in a ruck wrap.
If you're size 10-12, then a 3 would be long enough T. But, you may find that like Jen, you can get quite a lot out of a 4, especially something a bit thinner. I've had some 4s that I could ruck and DH in comfortably, which were great.
HPL - a peanut shell is a pouch sling. Some people find they're great, some don't. They do need to be the right size for you to wear. Some also find it tricky to get the newborn positioning right as they're not adjustable. Generally you'd do a cradle hold for a newbie, but if it's sized wrong, or if you don't get them in the right position, there is a risk of breathing issues if they get scrunched over on themselves. You coudl also consider an open-tailed ring sling, which is adjustable and you can wear your baby upright - tummy to tummy - which is easier to get the positioning right.
