The rear is 12 speed.
The new chain was made to exactly same length as the old one.
By "longer" yes I meant just by the amount of wear, so perhaps a few mm over the whole length, when stretched.
I have been trying to buy SRAM EAGLE parts; are there incompatible variations within this?
The "chain suck" is something which started only recently, and is on the old chain. The new chain just slips on the lowest 3 gears, under high torque (standing on the pedals).