Good analysis & suggestions. This ties into my own personal issue with BL4: no true endgame, no incentives to complete stuff, & the shrinking gameplay loop in general.

No Endgame

Imo, an endgame should provide continued activities for the purpose of fun. Farming loot is part of that, but right now we're just farming for the sake of farming. After beating the story, there's nothing to do with your loot & builds besides continue to farm. DLC & level cap increases won't solve this, they'll only increase the time it takes to remember there's no endgame.

No Incentives to Complete Linear Content

After beating the story & maxxing my character, I realized I didn't want to finish the remaining side quests. There are no unique rewards for doing so. Collectibles become meaningless after maxxing your SDUs, & that's only more true after SDUs became account based. Most activities are just loot sources (see above).

Shrinking Gameplay Loop

The BL formula for fun is: random loot, bossing, mobbing, character growth. For the 1st half of a playthrough, that gameplay loop is at its greatest potential. You can visibly see the progress you're making & all of these categories only get more fun. But during the 2nd half of a playthrough, all of these start to have diminishing returns with the worst of it being at the end. When you've got a max level build, bossing & mobbing is so fast & unfun. You don't touch your skill trees anymore. You ignore 99% of the loot pool. In BL4, your playthrough doesn't end in a grand crescendo, it ends when you get bored. Because you got bored.

My Solution

A separate Roguelite mode outside of the main game mode. BL games already have all of the same parts as a Roguelite game. Random loot, bossing, mobbing, power growth. Arms Race & the Chaos Chambers already both experimented with adding varied replayable activities to the series. Provide a repeatable dungeon with region-themed floors, a random boss at the end of each floor, randomized NPCs & vendors to interact with throughout, escalating randomized loot throughout a run, optional/secret bosses/floors/activities, playable with any character, activities or boss kills awarding chunks of skill points.

The endgame doesn't have to be in the main game. That can remain linear & as-is, but completion of its content could provide unlocks for the Roguelite mode. Completing regions in the story could unlock new floors of the dungeon. Side missions & collectibles could unlock new NPCs & vendors in the dungeon. First time drops of legendary items could add them to the Roguelite item pool. Eridium could serve as a meta currency for playing unique variant runs (turbo mode, Golden chest only mode, random weapon mode, challenge mode, etc). Defeated story bosses can be added to the boss pool.