@stageandgrace Thanks for the kind words!
Pretty common for total sleep to drop less than kiddo’s actual sleep requirement leading up to a nap transition. The sleep debt basically builds up until kiddo has night sleep disruptions (typically early morning waking), then you start the transition. Typically the more you are able to manage the sleep debt and hold off the transition, the more time your kiddo has to lengthen his WWs and the easier the transition will be.
In your case I think you can definitely do some 1 nap early bedtime days to 1) offset some sleep debt (TWT on these days will be short) and 2) get a feel for the longest WWs he can tolerate. It’ll be pretty easy for you because he sleeps in. So basically:
1) pick a day he sleeps in and you have control over his schedule, and let him sleep in as long as he wants past 8
2) offer first nap on the longest first WW you think he can handle, maybe try 3.5-4 hours, wait 15min before getting him after the nap to give him a chance to fall back to sleep
3) hopefully the nap will go 2+ hours, in which case you can bring bedtime up by 1-1.5 hours. If nap doesn’t go longer than 1.5 hours, it’s a sign that the WW was too long for him and he’s still quite a ways from 1 nap.
On days he wakes up before DWT, don’t get him before DWT but still do 2 naps, cap last nap to protect bedtime.