OP, there's really not much one can do with tiny babies beyond keeping them clean and fed.
'I hold them a limited amount compared to other mums'
They spend a lot of time asleep usually, also how do you know how much other mums hold their babies? There's not some 'good mum' amount of time to be holding your baby, as long as you are responsive when they need you.
'I will leave them to sleep or play on the mat (supervised) if not crying'
Sounds fine to me. Remember, everything is new to a tiny baby! Staring at a dangly thing above you on a playmat is totally mindblowing.
'after the nappy is changed, they’re fed and the cot is re made and a wash is on, bath done, I’ve got little left to give'
Also totally normal - babies this little are tiring and also limited in their interactions, they don't need constant stimulation, and, as I said above, just looking around and seeing stuff is stimulating them. They don't need to be being read Homer's Illiad in Cantonese, they just need your attention when they need you, which you are giving enough of. The whol idea of constant interface a stimulation with babies is really quite a recent invention - it wasn't a thing when I was a baby and I grew up with a very close and loving relationship with my mum (who had 2 older kids under 6 when I was born, so must have been pretty damn busy and not with me constantly) .
Similarly, there’s inevitable times they’re left to cry longer than they would be if someone else was with me in the house, for instance if they wake up when I’m in the shower
Babies will not be traumatised or damaged by you taking a little longer sometimes, they will know you are reliable enough and that is all you need to be. Please, please, please have the shower. It is important to take care of yourself. Life has to go on, stuff has to come out of the washing machine and so on - you are doing fine.