So if your BTC ever went through a segwit transaction, even if you are holding it in legacy format address, it would still be vulnerable to this hypothetical attack?

For example you are holding BTC in address 1xxxxxxxxxxxxxxxxxxxxx (1), then you send this BTC to address bc1xxxxxxxxxxxxxxxxxxxxx, then you send this to another legacy address 1xxxxxxxxxxxxxxxxxxxxx (2).

You are holding your BTC in legacy format, but it has a previous transaction in a segwit bech32 format. This means it's vulnerable to the miners stealing it?

Maybe if you send it through a couple other legacy format transactions it becomes unfeasible for them? (im just asking out of intuition since I don't know how this works tbh, but I guess the more legacy transactions in your BTC history after a segwit transaction the better, as in the further you get from it) or it makes no difference? because on that case It would be too much of a mess to survive. If segwit proliferates most people will hold BTC that went through a segwit transaction at some point in time except the BTC you are holding pre-segwit activation and freshly mined BTC from miners that I guess are spawned in legacy format and not bech32.

Also have you run the TRB software? does it do anything special to avoid BTC that has history of segwit transactions? or are they simply transacting with people OTC and never using any exchange?

Also is the HD format from Bitcoin Core considered safe? (https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki)

I think if you tried to run TRB node with your wallet.dat in HD mode it wouldn't work.

TRB client is probably very slow and resource demanding. Core client loads so fast compared to years ago when I used to run Bitcoin-qt 0.7.0 or something which was my first ever used client I think, so on this department it's an improvement.