You really want to make everything super clean with some IPA. You could also use some carbon grip paste to help things along, but you need to be careful you don’t end up with it getting where it shouldn’t and making things wear away that you don’t want to wear away.
FWIW I was quite surprised to see my 3T road stem had a specified torque of 13Nm on the stem clamps – I didn’t dare go that tight (I went to 12Nm). That was M6 bolts though, so the force from that would be equivalent to an M5 at about 10Nm (still “a lot” – probably quite close to yielding the M6 bolt even in quite high grade steels.)
Your steerer should be fine a reasonable amount beyond 5Nm but I would be concerned about the bolts themselves and the stem if that’s what’s specified (it’s not unknown to see cracks on Thomson stems).