@Scifiscream is your hair naturally straight?
I’d imagine they advise against layers simply because you like to dry naturally and your hair is fine.
Layering fine hair can make it look thin and stringy particularly if it’s long.
Do you have a lot of fine hair? Or is it thin and fine?
If you have a lot of fine hair then layering would be absolutely fine, just a long square layer to give it some shape would be lovely.
If it’s thin and fine then avoid layers.
If it hasn’t been cut in 18 months make an appt! Tell them an 1/2 and inch is all you want off.
If it’s working for you without colour then I’d leave it natural. Yes colouring can make it appear thicker but once you start you’ll be at it forever!
There are products that help it to appear fuller absolutely. Professional ones usually the best but to be honest if you’re not in to Blowdrying then just invest in a volumising shampoo and Conditioner.
Don’t bother with the styling products because most of them work best with blowdrying.