Okay, so you don't live in a "mild climate" but you think cool down issues are overemphasized because seeing issues often overwhelm cooling issues where you live due to thermals and your scope has plenty of time to cool because you don't start observing for several hours after sunset because of all the novices and traffic at your observing site?
How thick is your mirror? Mine is 2" (Obsession).
There is no one ever at my observing site after dark, so I don't have to worry about that. And there aren't any thermals nearby as I'm out on a small steppe and it's quite a few miles to the nearest mountains. But I do have to contend with high altitude seeing issues as I'm right under the jet stream.
I too often set up a four inch refractor (FC100DF) next to my 20" dob. Many (maybe most) nights the refractor does better than the 20" on planets and double stars due to cooling issues with the big mirror that renders planetary views complete mush and makes tight double stars impossible. I installed corner boundary fans but that didn't seem to help. The only time I've gotten good planetary views out of the 20" during the summer planetary season is early morning right before the sun comes up when temperatures stop dropping and start rising. Then the 20" gives outstanding planetary views.
The 20" can also give good planetary views after a few hours of cooling during seasons other than summer when the nights are longer and the temperature swing is less.
Since it takes unique circumstances for the 20" to give good planetary and double star views, I use it almost exclusively for DSOs since most DSOs aren't as affected by by cooling as planets and double stars. And I can use my 4" refractor in town for planets and double stars since those aren't as affected by light pollution.
So for me, cooling issues are the main factor that determines where and when and what scope I can use for which targets.
You misread what I wrote. I start observing about an hour after I arrive at the site most of the time. About half of that time is set up, with the mirror running during most of that. The rocker box is wrapped in a blanket for protection on the way to the site, so it isn't cooling much until unloaded and the fan is connected. Lots of disruptions, a fair amount of planetary observing early associated with those. It isn't my most effective observing time, so I don't worry about it, and instead work around it, often arriving later.
Mine is a 2" thick Galaxy mirror in an Obsession. I don't know why yours has so much trouble cooling sufficiently or keep up, but mine does not. "Summer planetary season" in the Pacific Northwest might be part of the problem. The ecliptic is rather low in the Summer that far north (it isn't well placed here either), so the major limitation to seeing is unlikely to be the cooling of the mirror, but instead the problems of observing planets low in the sky which you won't have control over. As I have said before, this lower elevation seeing favors smaller apertures and refractors mounted up off of the ground.
My 20" trounces the refractor's views any time the seeing is good enough for planetary. I see that near sea level in the backyard, and I see it up in the mountains. I don't use the scopes much pointed at planets particularly low in the sky because of the seeing disruption (and to a lesser extent atmospheric chromatic dispersion.)
I find the jet stream far less of a factor here than local thermal mediated weather patterns. Most of the disruption appears to be relatively close to the ground, rather than the high atmosphere. Breezes have been the best indicator. When the surface layer becomes calm the seeing is sharp. When the diurnal patterns pick up, or some sort of change is happening (where I can actually feel the temp delta's in the mixing air), the seeing gets bad. Mush in the 20" is not great in the refractor either...that is what I see side-by-side when this happens.
This can be particularly bad up in the mountains where thin layers of katabatic winds can dominate. It is a major problem at my darkest site at over 8,000 feet. DSO observing isn't even worthwhile in such conditions when the disruption is severe. I can have some wonderfully dark and transparent sky, and pack up because the seeing is so poor that 15 mag stars are little pseudo-galaxies. I make the call whenever I conclude that I would rather not record observations of blurred galaxies that won't reveal characteristic details. Sleep becomes a better use of the time.
Smoke in the air during recent summers/and early fall helped transform my impressions of the major sources of poor images in scopes. I had been puzzled in prior years why even lower jet stream speeds did not reduce naked eye twinkle in summer, and how the seeing was chronically poor in all of the scopes in the backyard. With smoke preventing me from observing DSO's, I was doing more planetary in the backyard with the 20". That revealed a surprising benefit to the smoke: thermal stabilization of the air. Twinkle would subside (for stars that could still be seen) and the images would become still in all the scopes. And as soon as the air began to clear, the seeing began to deteriorate in all the scopes. It wasn't thermal problems with the mirror (and I knew that already from the twinkle) but roiling layers of cooling ground level air that was the source of the problem.