Don't get caught up in Db "ratings". Most figures you see are nothing more than marketing. There is not an industry standard for testing, and it's known that some companies move equipment a little here or there to get better numbers. Not to mention, you're not going to hear the difference in the minor variation of Db - you're more likely to hear the difference in tone. If that really matters to you, then you'll have to find somewhere to shoot them all to see the difference.
What really matters for a suppressor is precision & repeatability. TBAC is the current gold standard (whether guys like it or not).
Don't get caught-up in the "modularity" nonsense. Swapping out endcaps or mounting is marketing and doesn't have a benefit. The ability to unscrew the front cap and/or rear typically results in a weaker can overall - there's no benefit.
For mounting, if you like the ability to jump between a suppressor and a brake, then get a mount (assuming you like the brake with the mount). Other than that, you are paying an extra $100+ per rifle for no benefit. I would also avoid adapters. If you're barrel is too skinny to mount a can - it's too skinny.
Most of us packing cans on a hunting rifle also consider weight and length. All my TBAC cans have weighed dead nuts exactly as advertised. I can't speak to the Sandman or Omega.
Length can be a factor when packing a rifle. I would avoid a k-can for your first purchase. Nowadays, the 7" length suppressors are really good, and I would only get a 9"+ for a rifle that I was not intending on packing.
As for cleaning, basically every single can out there is "user serviceable". As stated, centerfire can don't really need "servicing". One of my cans in particular has thousands of rounds through it and has never been cleaned. I can weigh it tonight to see how much carbon build-up there is if anyone cares. The last two 5-shot groups out of that can last week were ~5/8 MOA, each (with factory ammo). There's no real benefit to taking apart a precision rifle can.
For your first "precision rifle" suppressor, go with a 30 cal, direct thread, Gen 2, Ultra-7.