The main problem with pen style devices Ruby is that you have to seek ones with fixed voltages.
Most work like what you'll see described as "mechs". That means that the voltage starts at 4.2v when fully charged and slowly drops as the battery discharges.
Most of us find that mtl tanks work best around 3.7v so the coils can get a little burnt especially if your choice of liquid is quite thick.
A 60% vg level is the maximum I personally like to run with in a mtl stock coil tank to ensure the best life.
The t18/22 are very good devices but the coil design required regular tiping to fully wick the juice as the coil is very long and the cotton can easily burn when the liquid level is low if using thick juice or chain vaping (something I'm guilty of).
The best two mtl stock coil tanks available currently are the innokin zenith. A fantastic tank with a wide range of coils and airflow choices. This works excellently in the coilfire platform kit. Ok it's not a pen mod, but it's a very small compact kit that fits in the palm of my hand.
The second choice would be the aspire nautilus 2s. A recent revamp of the nautilus 2 tank. The original nautilus was considered the best mtl stock coil tank for years and the nautilus 2s takes the same great coils. Again I'd look at a small mod to pair it with and I don't think you'll go far wrong.