Have you opened the primer flash hole to 2mm and run Magnum primers? Apologies if I’m playing an old record here.......
I’ve only loaded subsonic specific projectiles (Lapua 200gn) and with 9.8gn TrailBoss coupled with the above case modifications and Magnum primers have have always produced excellent results.
The advice I was given was to avoid projectiles not designed for subsonic use as you’d have too much surface area of the projectile being “driven”.
However, with the advent of 300 Blackout and other subsonic specific calibres, I’m probably way out of date.
Case in point, these 308/7.62 subs. Right side is my “normal” Lapua load, whereas the left hand round is a tad different. 6.3gn Autocomp behind a 178gn Hornady ELD. Case capacity is roughly half that of a normal 7.62mm cartridge.