Most of those can be avoided with a bit of foresight. Take Balrogs, for example: by the Third Age there weren't all that many of them left, and on the off-chance that you
do run into one, you can lure it into a trap made from one of those extremely deep pits that Dwarves are so fond of digging. Just make sure the pit is narrow enough that it can't spread its wings (if they have wings) and be careful that the pit doesn't have water at the bottom to break the Balrog's fall -- you may need to use bilge pumps. Perhaps consult with someone who managed to beat one in a fight; Glorfindel is still around, and making regular visits to maintain friendly relations with the Elves is just good business sense. And of course you should make sure that certain key sections of tunnel can be collapsed; you'll want this anyway to deal with the nameless monstrosities older than the world itself that you start running into at a certain depth.
The other threats are analogous. Giant spiders? Creepy but overrated. Someone with decent aim can one-shot most of them with a thrown rock, as demonstrated by a Mr. Baggins of Hobbiton, and their territory is a fairly small portion of Mirkwood. Dragons? Definitely an issue, but one that can be mitigated by storing your treasure horde in a maze of tunnels too small for a dragon to crawl through -- or, better yet, use multiple geographically isolated hordes. And again, the ability to collapse certain sections of tunnel would be both technologically unchallenging and very helpful here. Arguably it was the Dwarves' lack of innovation that led to them having such persistent dragon issues.