Sorry for my ignorance as I know this will have been covered many times but unable to find an answer through searching
I'm going to do a sump oil change on my td6 gearbox
I understand the oil should be around 35-40 degrees. But I have no diagnostic. Any idea roughly how long driving it'll take to get to the correct temp?
Also, oil and filter then wait a few weeks for just oil change.
Or
Oil then wait a few weeks, filter and oil?

No need to warm before dropping the oil...although running it for 5-10 minutes will warm the oil sufficiently to drain easy enough....even cold it'll drain nicely.

It is the filling temp that is more critical. no need to drive it as pure hydraulic friction will warm the oil nicely.....I have found that about 10-15 minutes of idling will warm the oil to around 40degC....no need for diagnostics...if you have a temp probe attachment and a suitable multi-meter you can use that, or an infrared thermometer etc onto the sump pan will give a reasonable result.....

I'd do the oil and filter this time around, then oil again in a couple of weeks....and if you have enough oil....do another oil change a few weeks after that.....the more times you do it, the higher the concentration of new oil to old oil.

It prevents 'New Oil Syndrome'......Transmission oils are very good detergents (they keep the internals of the box nice and clean) but if you flush out all the old oil and replace with entirely new oil, it has been known that the new oil can dislodge all the harmless debris hidden in harmless nocks and crannies in the box and dump it into the oil......this sudden surge of foreign debris can trash a box in short order.....so to prevent this, it is best to slowly dilute the old oil with fresh oil to lessen the 'cleaning' effect.