Plus miners are ready to run their machines for any job be it mining or hacking.
ASICs are built to do a single job. They cannot be repurposed to try to hack bitcoin addresses.
Is it possible (theoretically) to Brute-force multi-sig addresses?
As far as I know the only threat to our multi-sig wallets is the risk of being hacked.
Sure it is.
Legacy addresses are P2PKH, or pay to public key hash, with the public key hash essentially being the address we are all familiar with. When we talk about address collisions or hacks, we mean someone finding another private key which leads to the same public key hash, which would allow them to spend the coins. It doesn't necessarily have to be the exact same private key. There are far many more possible private keys than there are addresses, and so there are multiple private keys which will unlock any specific address.
The same is true of P2SH addresses. There are a much smaller number of script hashes than there are of possible scripts. Any script which hashes to the same value as your multi-sig set up will be able to unlock the coins contained on that address. So technically speaking multi-sig addresses are just as vulnerable as non-multi-sig addresses to an address collision, but since an address collision will not happen before the extinction of the human race, I wouldn't worry too much about it.