Hi,
Based on what you already have and the presumption of not wanting to duplicate too much, I'd suggest these options:
1. Magnus or 338 Ultra Gen2. For maximum suppression period. The Magnus in CB or HUB mode is only 9" long, same as an Ultra 9, but way quieter. Let's not forget that 1.5"x9" long cans were the "standard full rifle suppressor" size until recently. The Magnus CB/HUB isn't any longer than this.
2. Ultra 5. For very small/light. Ultra 7 is almost as light but suppresses better.
3. Dominus-K-SR or Dominus-SR. For a fighting rifle.
To answer your questions specifically:
The Ultra 5, 7, and 9 are sort of in one "family", the 1.5" diameter. The 5 and 7 are very quiet for their size (volume) and weight, and are probably the best suppressing cans per ounce out there. The 338 Ultra and Magnus are sort of in another family, the 1.8" diameter design class for full size cans. The 338 Ultra just shares the Ultra name due to history. The Dominus-SR and Dominus-K-SR were made specifically for semi and full autos like the MK18, other SBR's, 308 SBR's and battle rifles, and don't share the same internal baffle design pattern of the Magnus/338Ultra. The Magnus internals aren't better or worse or newer or older than the Gen2 Ultra/Magnus baffles, it's just that to get the level of suppression we wanted in the Dominus-SR (ie the length specification) and have it work well on gas guns, we had to go a different baffle design.
Ultra 7 vs. 9, I'd say a better question is Ultra 7 vs Magnus. In that case, Magnus for max suppression in a .30 cal can. The Ultra 7 if you want good suppression but more compact.
The SR mount is the best mount we make. It had the same accuracy/repeatability as CB/BA/DT but mounts quicker and has a secondary lock. This is the lock we submitted on the 338 Ultra SR that AI submitted to ASR with. The way it works it is impossible for it to be "off by one" click, like many mounts that have accuracy problems. The SR mount permits cans that are otherwise good for F/A firing schedules to be rated F/A on mount retention also. The CB mount is sufficient for almost all precision rifle and semi auto shooters. It has better retention than a DT setup. The HUB mount on the Magnus is an experiment for us and is intended for people who already have an investment in another mount system and know that that system meets their needs (ie, it's on you).
Yes, a DT can is more likely to loosen vs. a CB/BA/SR suppressor.