Delayed for what, approximately 2 years
That's not really what happened - there wasn't a solid planned release target until the 2023 date. They didn't commit to a release window early on (early access was only in October 2020, and was basically what they'd finished up to that point), but people pestered them for a hard date, and in mid 2021
Larian stated that 2022 was the earliest it could possibly release, but 2022 was by no means something they were committing to. Then in early 2022,
they said they wouldn't be able to release by the end of the year, but committed to releasing in 2023, which they ultimately did. There wasn't a two year delay, and even the second one was arguably not a delay (all Larian had really said was that 2022 was the earliest it could possibly happen if literally everything went right, and people ran with it).
were allowed to work without interference by WotC
There were probably contractual obligations ironed out well in advance, as well as the basic game structure (including leeway to modify tabletop rules/abilities, which... well, already existed in the D&D tabletop rules anyway, if we treat Larian like a DM here). WotC didn't (and still doesn't) really have a sufficient internal digital production system to enable micromanaging contracted studios directly, AFAIK.
the developer (Larian) said "We're done with D&D and WotC!", not because the IP owners (WotC/Hasbro) didn't want any DLC.
The "No DLC" thing is mostly just a Larian preference and not specific to BG3.
One of the big reasons Larian abandoned D&D (thus making DLC a no-go regardless of Larian's usual reluctance; BG3 is probably the closest Larian has ever been to willing to make DLC) was that literally everyone from WotC in the pitch meeting for BG3 - passionate people who had provided Larian with connections to WotC during development - had been fired by Hasbro before BG3 actually released. There was no real effort on the part of Hasbro to maintain their relationship with Larian through their rounds of incessantly "restructuring" WotC. Swen Vincke (Larian CEO) somewhat notoriously ranted about how "publishing companies" (he was talking in general terms, but also specifically about Hasbro/WotC, and he wasn't super subtle about it) kept bungling their connections and abandoning their own personnel to chase quarterly share price growth.
This is something Hasbro continues to do to WotC, despite it being one of the only things keeping them afloat. As in, as recently as
two months ago, literally the day before Hasbro's q3 2024 financial report. It's also interesting how the
presentation for that report blames lapping the BG3 release for the 5% decline in WotC's
total q3 revenue compared to the previous year (see slide 5), meaning according to Hasbro that decline alone wiped out the gains in MtG and then some. Whoops... maybe that wasn't a good relationship to neglect. Oh well, I'm sure they can just cost-cut their way back to better margins forever, right? Surely cost-cutting doesn't have negative long term effects that more cost-cutting can't solve... right?
But I digress.
Swen's rant was right before Larian announced they were done with D&D. It was a bridge Hasbro didn't quite "burn," but rather removed and refused to replace all the supports on their side for... dubious reasons.
Given Swen's personality and the team at Larian, it's likely they wouldn't have wanted to do anything D&D for their next game, but "we're done with D&D" was
much stronger language than I'd expect from Swen/Larian if the relationship hadn't soured permanently. Larian generally doesn't make definitive hard statements like that. It's clear the relationship soured from stuff on the WotC side long before BG3 actually released, which is a damn shame IMO. Hasbro left that bridge to rot, and Larian moved on.