I’m looking for a very rugged handheld. I checked out the Yaesu VX-6R and some of the others that are supposed to be rugged but they weren't rugged enough.
What's the application? Is this for amateur radio, or something else? Letting us know how you use the radio and what the environment is like would be helpful.
What I’m looking for is IPX8 water resistance and MIL STD 810 toughness.
IPX8 is pretty extreme. That's going to be a limiting factor. IPX7 is a bit more common. Unless you plan on going swimming with your radio, the IPX7 might make it a whole lot easier. If full submersion is needed, waterproof bags can work better.
810 doesn't really mean anything on it's own. It covers everything from fungus, gunfire shock, dust intrusion, water, etc. Then there are levels for each one. I've found a lot of amateur products will claim very vague "MIL STD 810" rated, with little in the way of qualifiers.
The only thing I’ve found this though is military and police radios. Could I buy something like a Motorola SRX2200, Tait TP9300, or Kenwood NX-5200 and use it as I would a Baofeng UV-5R?
"Baofeng UV-5R"? Again, we'd need to know what the application is. Comparing Motorola, Kenwood and Tait to Baofeng is sort of like asking if you can use sports car like you use your rusty old lawn tractor. Big difference in application.
Or are these radios made only for groups with full-time administrators that can set them up? I realize they're way more expensive but let's not worry about that for now, please.
Any 'real' commercial/public safety radio you buy will need to be programmed. That will require -expensive- software and programming cables. Some do have an option for "Front panel programming", but it's limited in what you can do. Also, the front panel programming will void type certification for some applications. Again, knowing what you want to do with the radio would help us make some suggestions.
I realize they're way more expensive but let's not worry about that for now, please.
I get that. However, we hear that a lot. I think some may not understand just how expensive we're talking about. Defining a budget will help. A new radio like the ones you mentioned, with all the accessories, programming software, cables, etc. can run north of $2,000.00 depending on how you have them spec'd out. If your budget won't allow that, going with a use radio may make more sense.