I went digging through my notes when I did my research on the G30 and I spent some time working with the service advisor who has been taking care of my current F10 550i before I made the decision to buy the car.
Biggest issue they have seen so far (as of 2/2020 when I made my decision) with the G30 is a slight tendency for it to drink coolant. He's told me I can come by if it happens and they will take care of it immediately.
He also mentioned that they had an issue with the active cooling vents for the engine not being durable enough in cold weather - an engineering fix was applied and it shouldn't be a problem with the 2020 MY and later cars.
Also, he said that since the G30 came into service in 2017, they haven't yet seen exactly what the long term issues with the car might be. Those will begin to play out next year.
I looked at the data Consumer Reports holds for the G30 MY 2017 cars, and it shows reliability issues with the body integrity (squeaks, water leaks, etc.), in car electronics, climate control, fuel system, and average performance with the drive system. Those issues all improve as the model years advance, likely (IMHO) because the MY 2018, 19, and 20 cars haven't aged enough to show the wear. Also, CR only shows the aggregates across the range of trim levels, not for the M550 in particular.
WRT the extended warranty, I recall the prices my CA quoted me as being really steep (I can't find my notes, but I want to say $6-7K and up to get it to 7 yrs and 100K miles).
If I remember correctly, you can extend the warranty anytime up to the original warranty's expiration, so you can wait to decide.
I'd suggest some internet searching:
Here's a couple of reviews of the BMW extended warranty:
https://www.thedrive.com/reviews/268...ended-warranty
https://www.bmwusa.com/explore/bmw-v...nty-books.html
The warranty brochure is here:
https://www.motor1.com/reviews/39455...nded-warranty/
Also, I'll offer the expected timelines for certain maintenance items my service advisor shared with me for the G30 M550i:
Oil Changes - every 10K miles ($180)
Spark Plugs - 60K miles or 6 years (sorry, didn't ask the price)
Brakes Flush and Cabin Air Filters - Every 2 years (didn't ask here either)
Engine Air - Every 3 years (or here)
Brakes (front) - $1,200 for the Pads and Rotors - Rotors should last 80K
Brakes (rear) - $1,100 for the Pads and Rotors - same rotor wear interval
Is the extended warranty for the G30 worth it? I think you need to do the math based on your expected use of the car, your projection of the repair expenses, and how long you plan on keeping it. I'd also look into the viability of an extended warranty from a third party, but I don't have any direct experience with them to share.
Hope this helps you out.