Just my 2 cents and imho 2 points to be made here
You havent been doing too much polishing and lets not forget the Flex is a forced rotation machine. Its still a DA but it has more potential for you to do some harm to the paint. I would NOT advise you to get the flex in any way. The GG machine with Surbuf or MF cutting pads will be all you need and you get a little more peace of mind.
I would advise this:
GG polisher
2 surbuf pads cutting pads
2 MF cutting pads
D300 or UNO (smallest size available)
M105 (smallest size available)
2 pk MF finishing pads or 2 Green HD pads for finishing
Now to my main point I think you need to have a realistic attitude about these things. You havent polished paint in 3 years and even then you dont do this for a living. You need to stop expecting a flawless swirl free finish. If you go to the click and brag section you will find details by guys who have been doing this for a long time and believe me I stare at amazement at some of the results by guys like Thomas, Bob, Barry, Charlie, Marc and Jacob, John K, Philip Y, Scott, etc.
Realize there is a skill threshold ceiling you wont break through for some time. It is one of the biggest lies told in this industry that "With this new product XXXX you will be able to detail like a pro"...you cant. Its just that simple. No product will enable you to correct and polish down your vehicle to a full correction finish. ENjoy the process of reading the info on this site, asking questions to people on here, and in the end doing the best job you can iwth your current skill level and understanding.
Once you are done with that first "welcome back to polishing" detail, go voer the car with a fine tooth comb. Take pictures of spots you missed and post them, post what products you used, and as much detail and pics as you can, chances are by the next time you are ready to polish again in a year or so you will do a much better job. I am not tlling you to set the bar too low, but at this point you arent a pro either. Just enjoy the process of maintaining your own vehicle.
I do a full correction on my wife and my car (if it needs it) once a year and I am still disappointed by it. Then again I am, and will always be my own harshest critic, as I am sure the very best and honest detailers here will tell you they are as well.