Screwed up 2020 Kia Niro EV inverter coolant situation, not sure what to do

I own a 2020 Niro EV and got the low inverter coolant alert at some point. I searched online and saw that this is a common issue and is covered under a recall. I took it to the dealership to have the service performed. The technician told me that there is a chance that the alert could come back. I've also read pasts posts on this subreddit that say the same thing. The alert came back, but unfortunately I didn't take it back right away. I can blame depression and a busy schedule, but it doesn't matter. Point is I screwed up and didn't take it back right away.

About a month ago the AC stops working. I took it to a local shop that works on EVs. They first tried recharging the AC coolant, which might have helped a bit temporarily, but not much. They then tried adding inverter coolant. The alert went away and the AC worked temporarily, but the alert came back and the AC has been spotty again.

I took it to the dealership and said that the inverter coolant needs to be flushed again due to the the known issue related to the recall. At this point, it's been about a year since they last flushed it. They now want to charge me over $300 to diagnose the issue. And the person checking in the car at the service center seemed quite skeptical that this was related to the recall issue. This fee would be waived if it's covered under warranty, but won't be if it's not. Also, they want to keep the car for 3 days to diagnose it. Is this common? I'm not sure if I should bother with the dealership, just take it to my mechanic, or what to do. I feel a bit stuck and overwhelmed by this.

Can anyone share their experience if they have run into a similar situation? Any advice, experience, or commiseration is much appreciated.