Reading through this, there's seems to be a bit of confusion regarding antihistamines. First of all, Benadryl. That manufacturer has several products on the market containing differing active ingredients. In the USA, Benadryl will contain the sedative antihistamine, Diphenhydramine. Generally speaking, unless advised by a doctor it shouldn't really be used on a chronic basis.
However, I believe you're in the UK Dante. The available Benadryl products over here will likely to be containing either Acrivastatine or Cetirizine which are both classified as non-sedative antihistamines and can be used on a long term basis.
Regarding Fexofenadine, the 120mg strength was deregulated from POM status at the backend of 2020 and so made available to purchase without a prescription. However, if Benadryl works for you, then I'd recommend to stick with that and consider switching to an alternative once your system gets used to it and so is no longeer effective.
Dermatographia or skin writing is a form of urticaria. Exact cause is unknown although general school of thought is that it's linked to a slightly over exuberant immune system kicking off an allergic response. Apart from taking antihistamines, avoidance of known trigger factors such as exposure to hot water, harsh soaps/toiletries, staying away from wolly or similarly itchy types of clothing may help.
One important thing to do, is to moisturise. Dry skin will make it itchy, therefore applying suitable emollients will hopefully benefit.
You mentiond that it's moderate, as long as the antihistamines you're taking works then all well and good. But for your information, for more severe cases, treatment can be stepped up with other options being available such as phototherapy or for more extreme hardcore instances use of MCA's.
Options are there to help you manage it effectively, which naturally of course a dermatologist should be able to guide you with.