How come no one ever gives the advice asked? Do you really think the OP does not already know the biggest factor is him?
There's a HUGE difference between say an costco bike and 5k+ carbon fiber bike with all the bells and whistles. Somewhere is the sweet spot for the non podium people.
Spend 1k more save 5 minutes. good deal. Spend 4k save 3 seconds not for the average joe.
Basically I think you want a used Z06 in the bike world. Not much goes faster than a used Z06 on race track for the given amount of money.
I don't have experience to answer you sobre.
The GCN did a video apr 2018 that might be of some interest to you.