I think a big part of the apparent issue is how quick his abilties effects are? Not the cast times, they're awful even with multiple yellow shards, but how the 2 2 4 combo gives like 7k guard before mods, easily getting up to 20k when modded. Keep the overguard cap of 15k that scales with strength but reduce how much guard the abilities give (to ½, ⅔ or ¾ of its current value?) and that will make people scream less often about how he is too strong.
His 3 3 4 combo could probably scale a bit less sharply, It doing 3x damage of the removed status effects that scales with strength is maybe too strong as well, and you could easily reduce it to 2.5x or maybe 2.0x.
But as others said there are a bunch of ways other frames can do the same stuff dante can but more easily. Just not quite as in your face as him? Styanax has guaranteed slash on each hit of his 4, plus overguard from it through an almost mandatory augment. But its slower to build up and kill things, plus the spears are harder to see. Styanax also has armour strip from his 2 and energy regen on his 3 allowing for easier spear spamming compared to dante's energy hungry 334 and 224 combos. Not to mention how overguard not being affected by damage resistance means it falls off surprisingly quickly in places like the circuit.
Saryn's spores or anyone with thermal sunder (especially titania for mobility) can kill much more easily than dante, as his 3 requires line of sight and tragedy (the '4' part of the 334 nuke), while not needing LoS, needs those slash procs to be able to do most of its damage.
His tome definitely needs an exilus slot though, maybe even a new tome mod exclusive slot or two as well. You need normal damage mods for it and that leaves precious few slots to put in a tome mod - severely limiting one of its main unique features.
