The wait between these sections has been terrible. At least the first bunch was like 8 episodes, the second one was just the equivalent of 3 episodes taped together. What's taking them so long?

No, it keeps on going.

The manga ended at least a year or so ago. The ending was... certainly an ending, yep.

Or worse, find a way to use "The Final Season" thematically in the narrative to retroactively look like they planned it all along (unless this is already explicit in the manga).

Either way, their use of "parts" since Season 3 has been super annoying and misleading. I hope nobody pulls this shit again.

I don't really think this tracks. The production committee gets final say on the branding and the number of episodes in each contract.

More likely, the production committee underestimated the remaining source material and thought they could wrap things up by commissioning an unusually long 16-episode season. It makes sense why they'd fall into this trap, I feel -- they'd just dumped Wit who had continuously given them scheduling problems and wanted to lock in MAPPA to get the thing finished in a single contract (which they clearly failed to do, since this season has now required 3+ separate production runs)

There's an interview from back in November 2020 which I think demonstrates their eagerness to conclude the series. It's not surprising to me that MAPPA were the only ones who thought what they were asking for was feasible:

Tateishi: "While season 3 was in production, we consulted with Wit Studio, and it was decided that The Final Season would be created by a different production studio. [...] Through many talks with Wit Studio, we understood that the continuation would be difficult, so we looked for a new studio. Kinoshita, Maeda, and I divided the searching duties among ourselves, but most studios turned us down. Everyone understood what big shoes they had to fill, and we were told by dozens of studios that they emphatically could not accept."

Maeda: "Of course, there were also those who told us that they had the desire to do it but just couldn't make it work in their schedule."

Tateishi: "The only studio that said they would consider it was MAPPA. When we talked to WIT about it, they said they could rest easy knowing it was in MAPPA's hands. The producers were in agreement, so the production shifted to MAPPA."

Kinoshita: "MAPPA took the matter firmly into their hands, saying, 'Attack on Titan should have a proper ending for the sake of the fans,' which also made a strong impression on us."

Maeda: "I think you can see their fighting spirit reflected in The Final Season's PV. The show is in good hands."

The producers also stressed that, for the sake of all the fans who have supported the series, they are determined to adapt the manga all the way until the end instead of concluding midway through. Kinoshita said that he has felt obliged to adapt the entirety of the manga ever since seeing the reaction to season 1 episode 13, where the Eren Titan seals the wall.

What do they fill the anime episodes with, with respect to the manga? Do the two have large discrepancies?

I just remembered how waiting two freaking years to get the ending that Got did made it so much worse than if the airing of final season hadn't been so dragged out.

Really glad I just got Aot over with quickly by reading the Manga over waiting however many years it's been since they announced the "final season" to get the ending that it did. It would have been another GOT type disappointment, since if I'm made to wait that long then I'm expecting something legendary like I did for GOT.

Am I seeing an obscure Biblical reference? In an anime community?

Lemmy really is the complete opposite of reddit in many ways. Instead of constant disappointment, I'm in a state of constant amazement.