why dont they just buy a sum and put it in their treasury for lack of a better term. I mean they hold gold, allegedly so why not btc? Ultimately ECB could help scale Bitcoin by holding it, and performing transactions off-chain for various clients?

And if they wanted to think really long term, they could just do what was done to the dollar all over. At first it was gold based, then slowly the gold was removed and all that is left is just a fiat note. So if ECB was smart(??) they would adopt bitcoin, become a custodian, and down the line be like "what we need this blockchain for, and voila we are back with fiat currency and in the hands of ECB again