I think I've, finally, settled on a configuration for Devious Followers-centered Skyrim that works pretty much as I want it to (as I laid out previously). The relevant settings being:

• Trade and Barter:  fBarterMax 8.00, fBarterMin 2.00
• Scarcity: 2x loot rarity .esp
• SexLab Survival: 50g toll, non level-based
• Devious Followers Continued: 500/50 base/level-based debt, 50% interest, 1000/1000 deal agreement/removal
• Perseids Inns and Taverns: 50% of default rental

I was on the right track before - using Trade and Barter to tune down the prices vendors were willing to give was the key. Offloading loot in town does kind of feel a bit unsastifying now as I get basically peanuts for most items sold, but it's probably the least unsatisfying compromise. To balance Skyrim's loot economy to my liking, it seems like one of three things must be true: recurring resource sinks must be tuned very high, the amount of loot must be severely reduced, or vendors must pay a lot less for loot.

But the payoff in the kind of game environment it creates is so worth it. For the first time in... maybe ever, I don't feel like I have to vigorously DM my own Skyrim playthrough just to not completely break the immersion. With a few small exceptions (no carriages), I'm finding myself actually trying my best and using all available (in-universe) tools to solve problems the game throws at me. It's also had positive effects on the Devious Followers side of things. When I took my first deal on the road to Markarth after realizing I didn't have enough daylight to reduce my 2K debt before the day's interest kicked in, it genuinely felt like a defeat.

Speaking of DF. I've been running with Belrand (spellsword merc at the Winking Skeever) as my Devious Follower this playthrough. He fits the role surprisingly well. My original intent was to hire him to clear Ahtar's mission, then replace him with Ahtar as a long-term follower, but I ended up deciding to stick with Belrand. He is kinda charming in an old-man kind of way and he has that balding, lanky hairstyle that fits the "creepy uncle" trope to a tee. That plus him being an old mercenary hanging out in a bar suggest a certain kind of dynamic that is quite different from the one my other characters had with Sven. In my mind my hedge mage character initially latched on to him as a sort of father figure, though (of course) the wily old merc had other designs on his hapless ingénue.

I also envision him as a bit of a drunk, which is a good excuse to handwave all the random times the follower AI will shit itself. Belrand stuck somewhere not helping out in this bandit fight? Oh, he's just shitfaced again. And with follower needs turned on in iNeed he just goes through all the booze I offload to him super quick, and he complains when he's out of drinks, which would normally be a little annoying but is a fortuitous bit of mechanical storytelling in this case.

(As an unrelated aside, I've been moving away from the original pure healslut character concept and going into Destruction magic, and goddamn. Having some offensive capability bypasses so much frustration related to the follower AI being dumb as bricks sometimes. Follower inexplicably ignoring an enemy archer taking potshots at him? That's okay, I can deal with them myself.)