I didn't mean to question your oncologist's expectations or advice. I was just pointing out that so soon after the cessation of ADT it is difficult to get much meaningful information from a doubling time per se. Your most recent six-fold increase in PSA brings you back to roughly the same level you were at in August 2017. If the next few month's testing shows that your PSA has resumed the same general trajectory it had in 2017 then you can expect your vacation to be brief.
My T recovery after two years of ADT was laggardly and ultimately unsatisfactory. After giving the recovery process over a year and remaining stubbornly hypogonadial I spent some time trying drugs that will sometimes encourage natural T production to resume. I tried Clomid/clomiphene (pills) and human hCG/chorionic gonadotrophin (injections) and finally wound up using a testosterone gel (which I still use). 9 out of 10 guys recover faster / more reliably than I did so I am a poor example of what to expect although with your T level only at 67 after nine months of vacation you may be slow to recover too.
One thing to possibly keep your eye on is the research on treatment of castrate resistant disease with suprephysiological T levels. The theory is that there is a range of T concentrations that that encourage prostate cancer to proliferate and that range has a bottom and a top. Most treatments for systematic prostate cancer try to drive T levels below the bottom of the range. This works for a while then castrate resistance sets in and the treatments become ineffective. Recently researchers have been looking at going the other way -- above the range. Several very small studies on men with asymptomatic castrate resistant prostate cancer have shown very significant responses to treatments with testosterone at serum levels much higher than is physiologically possible. There's still a lot of work to do to identify the best way to use it and to identify the subset of men it will help but there is definitely something there.
Just something to keep an eye on. It's a researchy treatment for castrate resistant men and it will be a while before it's ready for wider application. But you are still castrate sensitive with the first-line drugs so you won't be in their target population for a while yet either.
Here's a nice review article of which I can read slightly more than half: Supraphysiologic Testosterone Therapy in the Treatment of Prostate Cancer: Models, Mechanisms and Questions