I am constantly buying bikes to restore or refurbish - I thoroughly enjoy working on bikes as a hobby. But I have set a limit to the number of bikes I can justifiably own, and that number is four: a modern CX-style bike, a 1985 vintage race bike, a1963 3-speed, and until recently, a 29er mountain bike. Wife owns a cruiser. Even with those constraints, I have a total of six bikes right now, but three of those will disappear (one is for sale, one is designated for a grandson, and one is an experiment). In the end, I will always have three bikes that are my main rides, but I might have any number of bikes that will pass through the workshop. My wife is OK with this, since it keeps me off the streets. I guess I’d suggest keeping the stable small, but constantly work on upping the quality of what you have. Get the carbon mountain bike, but sell the other one - just keep pressing forward.