For the average MTB racer, how long do you think moving from XCM (Marathon) to XCO (crit) would take to be competitive again at UK Regional A - which I think is US Cat 2 or 3?
The former is long 4-12hr sustained endurance and typically below threshold racing, with the latter of course being short & punchy, so muscular endurance with high Threshold and VO2max over 1-1:30hr.
I race XCO, XCM and MTB 100s simultaneously and have been for 5 years. I do so now at an Expert/Cat 1 category. With MTB no matter the discipline there is always punchy efforts required, so I don’t think you’re far off. You should have the endurance from your longer efforts which is good, since that takes the longest to develop.
Short Power Build and XCO Specialty will get you to where you need to be. That combined with racing as much XCO as you can, you should be able to get an understanding of what’s required and gain valuable experience. The big difference, the all out effort for the first 10mins or so then settling into a hard effort.
I remember when I was racing my XCO series, typically 10+ races a year, I would look forward to the start of MTB 100s because the starts were so much less demanding.