Slop.

I think it's a lot more organic than you've laid out. Also no idea what you mean about "tagging onto D&D and other nerd subcultures", but the 40kRPG games (and Fantasy RPG even moreso) long predate the turn towards "mainstream popularity" you mention.

First off, there's Warhammer Fantasy Battle and Warhammer 40k. They're different systems with some similarities (especially earlier on) that drifted apart over time. Warhammer 40k was never at risk of dying out, but Fantasy was, and did. This was less anything to do with politics and far, far more to do with rules changes that alienated existing players and armies requiring immense numbers of minis that alienated potential players. Fantasy (and its "sequel", Age of Sigmar) is more or less irrelevant in the discussion of "Warhammer", and so the rest of this is just going to be about 40k.

Warhammer starts as a wargame in the 90s, and is therefore played almost exclusively by giant nerds. Compounded by the fact that you had to also paint all the minis and learn all the rules, and that the games took all fucking day, only fairly dedicated people really cared about it. As with all other things at the time, large numbers of these nerds are unpleasant people, open misogynists, etc etc.

Time passes into the 2010s, warhammer continues to exist vaguely in the background of pop culture, gaining some popularity through actually decent tie-in video games like Dawn of War or, later, Space Marine. A lot of the early leadership of GW leave and it corporatizes, which generally results in GW taking Warhammer more seriously as a product to be sold rather than a game to be played. Lore has become more established after multiple editions worth of codices and a mountain of novels, things that were originally jokes are now taken seriously, and grimdark reigns. Then gamergate happens. Large numbers of online nerds become nazis, and these same nazis latch on to the many problematic elements of 40k readily, such as any of the crusading factions or the general fascism of the imperium. GW themselves largely don't care or take a stance because they just want people to buy their toys.

2017 and the release of 8th edition is the start of modern GW. Many clunky rules from earlier editions (vehicle facings and armor values in particular) are removed in favor of easier to understand rules in a push to appeal to new players. 2020 and early COVID lockdowns sees a huge rise in the hobby and warhammer's profile in general. Multiple hobby and lore youtube channels all start now that people have nothing to do. GW finally issues a press release saying that Warhammer is for everyone and that bigots can leave if they don't like it. Bigots largely didn't leave because they like larping as fascists, and GW's still happy to take their money so long as they can keep the swastikas hidden until they leave the store.

Generally, Warhammer remains a "politically neutral" game about the glorious fascist reich killing the evil aliens, but occasionally they'll publish a novel with a queer character. Corporate woke, as you point out.