I have several 35 cal rifles in various chamberings. The issue with them is bullet bc is relatively low, mid .4 at best unless single loading long bullets. 200g are .3-.35 ish. Next build may be a tikka in 35 Whelen.
Imho 8.6 is a solution looking for a problem. Subsonic performance is really all about bullet weight as you can’t increase energy with speed. Also bullet drag is very low so bc is less important. many other cartridges can match or exceed its energy with bullets designed for sub sonic speeds. 458 socom, 45-70, 450 bm and they are factory loadings. The 450 bm also fits in a ar-15, not ar-10.