WarhammerTV is (a) cheap and (b) not GW’s main product.

The 9.99CAD is for WarhammerTV & Warhammer Vault & the exclusive miniature & unlimited army lists in the AoS/40k apps & a discount on merch.

The audience (i.e. people paying that money) is tiny compared to, say, Netflix.

So it isn’t bringing in a lot of money so GW isn’t motivated to spend a lot of money on making the app to view videos actually good.

They are a miniatures company who make most of their money from miniatures. They are keeping their focus there and most diversification is, I guess, usually one guy’s pet project rather than a major business move.

I'm not really familiar with RE9, but a some surface level research suggests that (mechanically speaking) mutation is just "the abilities of bad guys change mid-fight".

That seems like it is easy enough to bolt onto more-or-less any game.

You just need a variant stat-block for the enemy and something to trigger it.

D&D 5.5 has a bunch of monsters with abilities which change when they are on half hit points but you can use other triggers such as "is the target is a spell" or "the GM decides the players are having too easy a time and spends a Fear point".

The trick is to come up with mutations and associated mechanics which fit the narrative.

Amber Diceless which compares stats with GM fiet based on the situation.

Everway which also compares stats but if things are close has the GM interpret a picture card.

DramaSystem which is designed for PvP play and trades tokens for conceding a scene (and multiple tokens can be spent to force another player to conced).

Fiasco which does use dice, but not (as random number generators) to determine outcomes. A player, on their turn, can choose to establish a scene or hand that responsibility off to the rest of the group. Whoever doesn't do that picks if the outcome of the scene is good or bad for that player's character (subject to the availability of good or bad tokens in the pool). Second editor ditches the dice entirely and adds cards instead, I haven't played that version.

For the Queen isn't a traditional RPG. It provides a card deck that asks questions about characters. Figuring out the answers lets everyone learn about all the characters (including their own).

I can't speak from experience, only observation.

GW put out a game called Inquisitor which was the closest thing to a 40K RPG they published directly. It was a 56mm game. It's been a long time since it has been supported, but it has lived on in the community under the heading Inq28 (using 28mm). https://www.reddit.com/r/Inq28/comments/182y7xi/where_are_the_rules_for_the_inq28_game/ is probably a decent place to start looking for rulesets, and https://28-mag.com/ has a lot of adjacent content.

In GW's current rulesets, Necromunda is probably their most narrativey campaign game. https://start-warhammer.com/necromunda/

If you want an actual RPG then C7 publishes two games. Imperium Maledictum and Wrath & Glory.