It's interesting to note that this reluctance to utilize Bart more in the Selman seasons is coming off of perhaps what was the nadir of Bart's character: seasons 28-31 or so. In Season 29, we have multiple episodes where Bart is shown to be loser with no redeeming qualities, unwanted and personally disliked by his own family. Season 31's Go Big or Go Homer, in which Bart is viciously mocked, laughed at, and then cries miserably in front of his family in reaction, was probably Bart's lowest moment since wetting his pants in public in Love is a Many-Strangled Thing (the lowest moment before that being The Boys of Bummer). "Bart the loser", unsurprisingly my least favorite characterization of Bart, seems to be something the writers really leaned into recent seasons, and Bartless was, to me, a repudiation of it.
Yeah, I do think it may stem out of how horribly the writers treated Bart around the late 20's & early 30's (speaking in terms of seasons, mind you): That good for nothing loser approach with him being treated as worthless, useless, pathetic, mentally deficent (even by his own parents, mind you) and having nothing good to look forward to in his future (and yeah, let's not to mention his meaner, unlikeable side being grossly exaggerated, treated as a sociopathic manipulative devil child enjoying causing mayhem: Remember toddler Bart stabbing Kirk in the hand with a fork and pouring salt in the wound? Yeah, I think we all would rather forget that nonsense).
That scene in 'Go Big Or Go Homer' was really the kicker that really summed up how little the writers and showrunners thought of Bart back then (and seeing Bart literally lose it and just break down crying all night long really upset me when I first saw it). I mean, they probably didn't intend for it to look as bad (but it did), but they went too far (in my opinion) and the way they had been writing Bart for several seasons made that scene come off as in bad taste, especially for the Bart fans out there who wanted them to not continue the nadir of writing for him as a character, feeling disheartened about this. The timing of that "joke" was not good, to say the least.
Truly the worst Bart characterization (good riddance!) and much like you, I thought that 'Bartless' (lovely episode by the way, absolutely a S34 favorite of mine) was an affirmation that Bart still matters as a character and has a place both on the show and in the Simpson family itself: That episode, to me, actually came off as an apology for all those times they treated him so poorly (even though I'm not sure that was the main intent by the episode), but it still came off as a strong and interesting plot with a lot of heart.
But the question is where to go from there? They got a lot of mileage out of almost making Bart the Meg-like butt monkey of the family, and while they ostensibly will steer clear of that in the future, they seem to be having trouble with coming up for an alternate use for Bart
Like I've been saying, they could tap into that darker sibling conflict side of his and Lisa's or, well, just have more of his pranks and shenanigans and causing general mischief. While the more positively-inclined portrayal he's getting nowadays is good and all (again, so thankful we're not getting that worthless loser Bart portrayal anymore, which was an insult and mockery of the character), they need to bring back some of the edge of the character from before and not play him so safely, so the abeformentioned are ways they could take him.
But like I've been saying, they are clearly way too content with Vanilla Bart and prefer to use him suprisingly little these days (or play up his negative sides a bit too much, like yeah, we saw with the scarily incel-like Bart in 'Ae Bonny Romance'), but there are still instances of them putting him to better use ('Game Done Change' was one that worked out decently, having that mischief-maker Bart again and kind of a fun teamup dynamic with him and Skinner, his ol' school nemesis) but those are far and few between.
It's also worth pointing out that Bart's original role in the show was intended to be counter-cultural, but the show has become increasingly institutionalised over time and he's probably the most anachronistic member of the family now. Additionally, most of the writers on the show are 50+ years removed from being 10 years old and have often betrayed their ignorance of modern youth culture.
The thing is, even though the show has lost its counter-cultural streak and gone a lot more institutionalized overall, there is still more than enough room for Bart and his bad boy rebel archetype, even if he look more anachonistic these days (and they could do more with his more creative and inspirational sides, taking some cues from 'Bartless' and its opening, where he inspired little kids to read).
Sure, a lot of the writers on the show are more or less out of touch older fellas and know and/or understand little of youth culture these days (though they do seem to try harder with Lisa, unlike with Bart, so there is a clear pro-Lisa bias, I guess due to them loving the character), but they could still easily find more usages and roles for Bart that ring true to his character, without defanging or milking him down or exaggerating his negative sides (like they loved to do back in season 28 through 31), but they simply aren't interested in doing more with Bart than they are currently doing, which is a shame. And they do prefer do stuff with Lisa, even giving her roles that would kind of make even more sense with Bart, but he's left playing second fiddle or with an even more minimized role.
It's like they just ran out of ideas of what to do with Bart, kinda revealing he's not a favorite character of most of the staffers.