fair(ish) points, although a tank company (even a small one) picks up blast markers far slower than guard or even marines, 4+ RA means it takes 4 hits to kill a tank, that's 4 hits to cause 1 BM, wheras that's 4 dead guard infantry (unless they're hiding in cover)
a SL tank company is only 2 models bigger than an upgraded tank cataphract, which comes in 25 points cheaper and has slightly better firepower on a per-tank basis, I can't see a problem there, you have crusader robots as cheap throwaway activation count boosters (much like IG have sentinels) and thanks to your list structure (something I really like) you aren't restricted to being forced to take a 3rd company to open up extra support slots
I really can't see a sensibly built army using the original list in the OP being much more fragile than a SL army
In my opinion the marine comparison is a total apples-to-oranges situation, totally different armies and playstyles, and although the ATSKNF rule was added to make marines viable, there is
so much gumf written about how totes amazeballs marines are in all the army books and codexes and black library books to justify it.... from the background, a single marine costs about the same in imperial resources as a million guardsmen, and is widely regarded as well worth the investment..... they're artificially weakened in game terms to make it somewhat balanced I feel

I think you could get away with some kind of rule which ignores the -1 penalty to rally tests for enemy within 30cm and probably stick a spirit stones type upgrade in there to provide a formation wide
leader ability.... surely that's enough! never rallying on less than a 4+ is pretty solid, and I can provide numerous examples where not having that has lost me a game when using steel legion.... built-in leader would be useful to avoid them re-breaking when a single ork shouts 'boo' at them too....