The issue you're having likely stems from the REPAIR TOOL being overpowered, not the tanks.
Tanks are squishier than ever before in this game, they're paper mache if you know how to hit them. This is an issue. However, the repair tool is BROKEN overpowered, legitimately appalling how strong it is, so if there are a couple engineers on a tank, it is, like you said, borderline indestructible.
Paper mache tanks are really punishing, and broken repair tool leads to wildly inconsistent and unpredictable engagements that also lead to a lot of frustration for players who feel there is nothing they can do to damage one. I've had 2 minute long tank battles because neither of us can kill each other with all the repairs and we just give up and start farming each others' infantry. That sucks and is boring and lame.