this is more of a consequence of highlander being a bad game mode. you're taking the one class who relies on surprise the most and making it known that he'll be in play 100% of the time, it's like intentionally shooting yourself in the foot. based off of that fact alone spy will always be a bottom 2/3 class in hl, you just have to accept that it's an unfixable flaw of the mode.
honestly the only reason u can do anything as spy in hl is cause having 9 apes in mumble is enough to cause enough noise/distractions to actually get kills. but if your team can't apply any pressure you're nearly dead weight because, again, enemy team always knows that there will be a spy. barring an enormous class rework this will always be the case.
(also i would argue engineer is worse in hl, literally the most broken unlock in the game is allowed just so they can actually pretend they're doing something)