Alright, here's my ultimate crackpot theory that is most likely not true but is just plausible enough for me to share it:
The Switch Pro is what caused Golden Sun to be delayed and Mario Golf to be made.
Hear me out: we know Camelot self-funds their games, they have for decades. It is quite possible, as many of us here have theorized, that Camelot saw the demand for Isaac (and therefore Golden Sun) on the Smash Ballot and decided to begin work on a new Golden Sun game (remake for new game, whatever, doesn't matter in this theory). Mario Tennis Aces was Camelot's biggest success ever, it sold more than even the Wii's Mario Tennis. Therefore, they had quite a bit of money to throw around and Golden Sun was a passion project they were allowed to make thanks to the success of Mario Tennis.
So, they began work in mid-2018 or so, around the time Aces was released (and work on the game was passed to Nintendo's internal staff, as they did all post-game content) with Nintendo filing a brand new trademark to go alongside the game in August (as we know they did). Development was proceeding roughly (it would be Camelot's first HD RPG, after all) until around perhaps late 2019 or early 2020 or so, when Nintendo told Camelot about the next Switch model, a more powerful model that I'm calling the Switch Pro. Now, HD development, and especially HD RPG development is hard and very, very expensive. Camelot was probably already running low on funds and the news of needing to develop for an even more powerful system (possibly even optimizing the game for it) probably spooks the Takahashis into wanting more funds for the game. So, they halt production on Golden Sun temporarily to get more funds for the game.
What game can they make that wouldn't be super expensive but could lead to a big profit? That's right, Mario Golf. They already have the engine made for Tennis, and by collaborating with Nintendo's internal staff as well, they can get this game out the door as soon as possible. Coronavirus certainly throws a wrench into that, which is why the game was unavailable for the Mario 35th Anniversary Direct. Camelot's younger staff probably also needs help learning the ropes of RPG development, so they implement some RPG-like mechanics into Mario Golf so those staff members can learn the ropes and make Golden Sun's development go smoother.
Mario Golf is set to release in July 2021, which would be about a year and a half from when I assume it started (giving GS about a year and half of development as well if I'm correct) which makes sense, given that was about how long MTA took and that required a whole engine to be built. Even taking into account the couple-month delay the game would've taken due to COVID, the time to make still makes sense, especially if Nintendo helped like I think they did.
In conclusion, rising HD development costs forced Camelot to hold off on GS' production until they could acquire more funding and thus Mario Golf was born. They took the opportunity to implement RPG mechanics in the game to help their staff learn not only HD development, but also RPG development specifically for Golden Sun. Admittedly, this doesn't necessarily require that the Switch Pro is involved, but ultimately my theory hinges on HD development costs exceeding what Camelot budgeted for and them requiring more cash flow.