Because Philips license the HA technologies from William Demant, which owns Oticon, Sonic, Bernafon, they all share the same frequency lowering technology. This frequency technology is not based on frequency compression, but instead is based on frequency transposition and composition. Anyway, the point I’m trying to make is that I use it on my OPN 1, and to my surprise, it does not seem to affect how music sounds to me much at all, and I’m also an amateur musician. Another pro guitar musician on this forum (@SpudGunner) who wears the More 1 also uses the More frequency lowering without noticing any bad effect due to it as well. So it may be worth a try if you’re curious, although your kind of hearing loss is not severe or profound enough in the highs to need frequency lowering.
Anyway, if interested to try it out, you can simply make a copy of your P1 program into P2, then enable frequency lowering on P2 only, that way you still don’t have to live with it when in your P1 default, but it makes a nice A/B comparison without/with frequency lowering when you toggle between P1 and P2.
I’m not trying to talk you into using frequency lowering, but I was thinking that if you use the lowest configuration where the destination region is between 1.5-2.4 kHz, maybe in P2 have the High Frequencies option ON, and in P3 have the High Frequencies option OFF, it may be helpful with your debugging the screech issue, to see if the settings in P2 and/or P3 as set above helps resolve the screeching issue or not. If not in P3, then at least you know that the screeching is not happening in the source region (between 4-7 kHz).
And while you’re at it, try listen to some music to see if it sounds weird to you with the FL or not. Also see if you find it helpful with speech or not as well. Like I said, your hearing loss doesn’t warrant a need for it, but it can be a useful tool to try out, if anything, for debugging or just to know how it sounds like.