seems to do pretty well against majors, though using it on red bars seems like a waste of ammo given you can only hold 15

Yeah. Its like going after a mouse with a baseball bat. Its major overkill against red bars.

we need a catalyst to change it from 3-round burst to 1-round burst

Disagree. The three round burst makes the weapon versatile. You can either use it to attack a single high priority target.....or you can sweep it across an area to taken down trash mobs if you're about to be over run.

A “point defense weapon” ? What does that even mean?

You have three kind of enemies in the game. Those that stand off at either mid-range or long-range....and shoot you. Then you have enemies that will rush you and try to melee you. Lastly, those that use a combination of the two. A "point defense" weapon is one that is short-to-medium range....and does a LOT of damage in a short burst. Its purpose is to allow you to stand your ground, and defend a specific location on the map from any enemies that want to push you off of it.....or kill you where you stand on it. The game has a lot of Tier 2 enemies (Captains, Knights, Minotaurs, etc) and mini-bosses that will rush you. So weapons like shotguns, fusion rifles, swords (and now sidearms after the buff) that will allow you do enough burst damage to put these enemies down, and allow you to remain where you are. Bastion is VERY good in this role. It does a TON of damage to those mid-tier enemies at short-to-mid range. But if you stand off at long range and try to use it as a boss killer (like one content creator who shall remain nameless did in their initial video)? Yeah, its going to really underperform compared to weapons like Izanagi's Burden. Because it wasn't designed to do that. But when you compare it to weapons like Lord of Wolves, Telesto, Merciless, etc? The Bastion is VERY good.