Let me tell you that it wouldn’t going to be easy to find a good balance here. You have wiggle room, but you need to balance skills in a way that you neither invalidate the looting experience on the first half of the playthrough nor make the skill bonuses too top-heavy or else people will not do any hybrid builds as those naturally wouldn’t get as many high tier skills for IB.
There are some skills that would lend themselves to this as they need buffs anyway, like Explosive Punctuation, which you could give a static +20% buff to IB’s hard point damage. Or Why can’t I carry all these Grenades? and Torgue-Cross-Promotion, one of those skills is just bad and the other seems to be disliked for it’s inconsistency. Forge could also just have a bonus to IB’s damage slapped on top, encouraging it’s usage. And Force Feedback would also work. If Moze scores a critical kill while in IB, IB’s damage could be increased by 20% or so for a while. No massive bonuses, but overall those would add up the more you level up, rounding out to the build as a whole. Also, no more multiplicative bonuses as those throw off the balance of something like IB very easily.
Also, I would for sure tone down the Flare com as it is just a bit ridiculous. If you lower it’s bonus from up to +100% damage to up to +75% damage you’d keep it intact and still have it be less absurd. And last but not least, all “illegitimate” class mods, like Heat Exchanger with more than 1 point in Feature Creep, should be patched out. Have the game reroll the skills of those class mods once you load into the game, so that players don’t loose the passives, but couldn’t get totally extreme bonuses that weren’t meant to be possible in the first place. I know it will suck, I use one of those as well, but it’s just not how it was meant to be. But as it’s possible to do it warps our perception of IB’s damage potential.