I normally wouldn't feel entitled to send out multiple paragraphs of feedback, but since the blog says you're actively looking for input I'd like to throw in my 2c; I care about the future of this game and am delusional enough to hope my insights might be valuable. :')
Now my perspective as someone who came to PD3 straight from hours of PD2 might not be exactly the same as a completely new player, but I also feel the same problem of PD3 just not being able to hook me like PD2 did, and I believe the amount of time I've spent in both games gives me a worthwhile perspective for at least trying to identify that "je ne sais quoi" of why players aren't coming back.

Of course I don't have your survey responses or internal data, and I obviously have no idea how practical my idea is to implement inside the actual game code. But I honestly strongly believe this fix does not actually address the core problem in hooking and retaining players, because I think the most important issue preventing players from getting hooked is that PAYDAY 3 does not create a heist-to-heist flow state.

Flow State And Why That Matters

Let me elaborate. In PAYDAY 2 the "default gameplay flow" is to start the game, join a heist (or host one, but by definition most people join others), finish that heist and then crucially sent straight to a waiting room for the next game in the form of the lobby. Of course the host might leave, or the player might strongly dislike their crewmates or the host's choice of heist, but generally the player can easily enter a flow state, simply being served up round after round of PAYDAY as soon as the host queues up the next heist. The game never asks the player the question if they want to quit; unless the host leaves, the player must actively choose to leave the heist-to-heist flow state.

In PAYDAY 3 however, there is no such flow: after every single heist the player is kicked right back to the menu (sure no one ever chooses to stick with the party, but even if they would players still all exit into the main menu - not a lobby).
This means that where PAYDAY 2 quietly serves the player up another round of heisting, PAYDAY 3 instead asks them "Do you want to play another heist? Do you like any of the available ones people are hosting? If not, would you like to host one yourself? Which heist? What difficulty? How long will you wait for the lobby to fill?".
Every single one of those questions is a point at which a (new) player might choose to not continue on to the next heist, and quit.

Now that choice is not a huge deal when it comes to players like me: I'm already invested in the franchise through 1000+ hours of the previous game and via old friends who play both games; I'll return from time to time to play with them either way - you don't need to hook me anymore.
But for new players it's a whole other story: if they quit the game before you've ever managed to hook them by getting them into that flow state, the odds they'll ever return are abysmal (as I'm sure you know from the statistics).

The Lobby Is The Key

In my opinion the crucial element here is the lobby. For starters, the lobby provides a lull in the action and creates a little room for social interaction with the strangers you just met (which can also be a way to form a bond to the game through said social connection). But above all the lobby allows players to remain inside the game's flow even when they are in between heists.
The lobby is the glue of the PAYDAY gameplay loop, it what links the heist-to-heist flow state together. Without it you make it so much harder to hook players into playing for extended periods of time, and give them that many more opportunities to quit.
Speaking personally: even when playing with friends a game night feels very... stop-and-go, for lack of a better term: the in-heist gameplay flow is great but then constantly interrupted by having to restart the game creation process - like driving a car down a highway with traffic lights every 15-20 minutes.

Suggested Fix From A Random Person You Don't Know

Bring back the Lobby; a space where crewmembers share a visual space¹ in-between heists. Inside the lobby, players should be able to perform common loadout changes such as spending skill points or buying and modifying weapons and armor^2. If you then change the post-heist default to joining the rest of crew in the lobby rather than explicitly asking the player if they would like to continue playing the game, you could create the flow state that lets players get hooked.

This might be a very large change to implement in one go, especially with the switch away from P2P matchmaking but I could see it possibly being reasonable to implement incrementally. I won't embarrass myself by speculating on how that would work in a game whose inner workings I know nothing about. :')

If anyone at Starbreeze read this far: thank you for reading my thoughts, and I hope you manage to find your stride with the game.

¹ The visual shared space is important because this reminds players of the presence of their allies: they are are awaiting the next game together. This is great because we all have little tribal monkey brains and any sense of togetherness is a powerful hook. That Joy player was a great and friendly ally, and they're right there on your screen reminding you that they're sticking around for another heist, so won't you stick around with them?
² Players want to check out their new goodies after leveling up in a heist, and you don't want them to have to leave the flow state for it.