One part of the DeLisle was that being 45acp, it could be
very compact, and lightly built. It was also very quiet by virtue of the small volume of gas produced by a pistol round, combined with a silencer that was even bigger than rifle-sized. To be honest, all 300BO suppressed jobs work similarly; the round has a small powder volume compared to most other AR loads, and the cans are still full rifle sized affairs. I suggest sticking to the same concept, at least, and leave more powerful rounds out of consideration.
I do think a bit more power than 45acp would be advisable, since the DeLisle was for people, and your gun is more likely for paper or game animals like hogs. They deserve a bit more 'authoritative' demise in the interest of avoiding needless suffering, even if it ultimately makes the gun a tad louder.
I think 45 Win Mag would be interesting; could be loaded down to ACP levels, as well as much heavier subsonics, as well as very powerful supersonics of various weights, and still be formed from common NATO brass like Blackout. You'd still retain a very short action, too.
As far as rounds better than Blackout for suppressing, of course there's better offerings; the BO was optimized for the AR platform, so if you aren't similarly constrained, it's compromises come into it. Generally, smaller caliber rounds suppress easier, but you also have a hard time getting them heavy enough to hit strongly as a fatter bullet, to say nothing of the increasing difficulty in stabilizing those super-long needles (and also ensuring they still remain effective upon impact). 30 cal seems to be the inflection point beyond which it gets really hard to make a subsonic projectile worth the bother, and above 45 that fat hole in the can and fat bullet start to be limiting factors in how quiet you can be at the muzzle and in flight. There's a similar inflection point regarding needed suppressor volume as well, that makes cans for rifle rounds with more volume than 223-variants get big really fast to attain the same decibel levels.
I'm sure somebody has charts that relate sonic-velocity max stabilized bullet weight by caliber to terminal effectiveness, and powder volume to suppressor volume for a set decibel level by caliber. I suspect they cross near, but not at, 45acp and 300 Blackout
TCB