Higher penning light tanks on average do not have faster-firing guns than mediums or heavies.
No tank should be surviving side-shots, yes.
The T114 is 7.7 (so rarely faces WWII stuff), is not faster than contemporaries around its BR (including MBTs), and its gun can be 0.50 cal-ed.
The M552 (76) does face WWII stuff somewhat often, but again it doesn’t have crazy mobility compared to other light tanks around it and it easily can be taken out by any HE shell (plus it only gets access to HE or solid shot shells).
The ELC bis does face WWII stuff a lot, but again doesn’t have crazy mobility, its turret cheeks can be 0.50 cal-ed, and its reload is not much faster (if at all faster ) than most medium/heavy tanks around it.
The AML 90 rarely faces WWII vehicles, can be 0.50 cal-ed, and does not have a reload much (if at all) faster than the tanks around its BR.
The Ratel 90 faces WWII vehicles often, but it can be 0.50 cal-ed, and does not have a reload much (if at all) faster than the tanks around its BR.
The only one that somewhat makes sense is the Ru 251, which should probably just move up to 7.7 (and even then it doesn’t have a faster reload than most things at its BR).
That really is dependent on the tank. There are really no light tanks in my experience that have few disadvantages - and to be honest, its usually that the high-pen light tanks can be penned by literally any MG in the game, have slower reloads than most tanks, and are still not that mobile anyways.
The issue with that is that the Jagdtiger is still not terrible in an uptier. It’s got enough armor to stop most rounds with or without angling, you just need to be more strategic around where you move (which to me is a fair trade-off).
That assumes that the current post-WWII tanks at WWII BRs are being kept their artificially, rather than their BR purely reflecting their performance. Even if BR decompression did happen, it would not drastically change the BRs of post-WWII vehicles.