Joe, we discussed in another thread about your Surdiac being very similar in design and operation to my old Franco-Belge 10-175. Top load, hopper feed to rear of fire bed, etc. I can give you some input from my long experience with the Franco.
The thermostat on the Franco went, I believe, from 0 to 7 or 8. I generally set mine at 3.5 to 4 on average winter days, as high as 5 to 5.5 on coldest days, and as low as 2.5 on warmer days. I had an MPD and a baro in my setup. Usually had my MPD about half closed, sometimes 2/3 closed on windy days.
Judging from your pix, your fire is burning as hot as my Franco would be on the 5.5 setting. When burning that hot, it would only maintain that temp about 4-5 hours, then start to cool down. It definitely needed a shake around the 5 hour mark to keep it hot. Sometimes, on single-degree or lower nights, I'd shake the stove at 11 pm, then set my alarm to get up at 4, to shake it again and keep it hot. If I left it go for 8-9 hours on that setting, there would be mostly ash in the firebox, and the fire would have started burning up into the hopper. In that state, there were usually enough hot coals that if I shook it down carefully, and left the ash door open, I could recover the fire.
On the average winter day when I had it set back to 3.5 or 4, the fire wouldn't burn as hot as your pix look (more unburned coals on top), but the fire would burn 8-9 hours, and a good shakedown would keep it going. On the warmer days, with the thermo set back to 2.5, I still only got around 9-10 hours of good burn, because opened my MPD more, so more hot gasses would be getting up the flue to maintain the draft.
Bottom line, this type of stove just doesn't have a big enough, or deep enough, firebox to maintain a really hot fire for more than about 5 hours, no matter what you do. The best I could do, after 28 years with two Francos, was keep the fire going at a moderately warm temp with a minimum of three shakings a day. If I wanted to keep it very hot, I planned on shaking every 4-5 hours.
Incidentally, the very longest my Franco went without shaking, and still was recovered, was about 17 hours. The wife and I left at 8 am one December day when temps were in high 30s. I'd turned the thermo way down to keep the fire low. Planned to get back around 10 PM but make it till just after 1 AM. Thought the fire was out, but found it had burned completely into the hopper, and about eight or ten hot coals were still burning--barely. Shook it very gently to get the coals back into the fire box with a little new coal on them, then left ash door open for almost an hour. The coals heated up and ignited the new coal. Didn't have the full firebed burning again till next morning, but I was still on the first match.