Agree with your Original Era choices.
I disagree with 5 - 17 being considered 1 era. The late tournaments (especially from 13 onwards) are extremely different to the earlier ones in terms of obstacles, competitor pool, atmosphere etc. I divide them as 5-8 (the peak era) and 9-17 (era of the all stars), could even be argued 9-13 and 14-17 can be divided.
Of your chosen period I think for the easiest, 12 and especially 16 should be given some consideration, 16 had the most clears of this period, with a lenient time limit and joke obstacles like reverse fly and inadvertently made the wall easier by giving competitors a long run up.
Completely disagree on 10 being the hardest. I find it strange you say 5-7 which brutalized the competitor pool is the easiest of this period but it was due to competitors not being used to the obstacles. The same argument can be made for 10's jump hang which competitors basically mastered in 11 and 12. I'd say 15 is the hardest and without weather being taken into account 17 is the hardest of this period.
On Shin Sasuke, I consider it to be 18-24, 25 - 27 was the Kanzen renewal, this was a declared new era by the producers and while 26 did completely betray the vision for the (bringing back old obstacles and upgrading them and combining them with the new to create the hardest course) era, it still feels a lot different to 18 - 24. For example,the slider drop, double salmon ladder, buffed stage 3 with the Ultimate Cliffhanger and flying bar. The competitor pool/results were also very different with an influx of US competitors who did pretty well and Okuyama and Lee dominating. The era also had a different atmosphere/vibe than Shin Sasuke, with the looming fear of the show being cancelled, which was seen an inevitable.
Of the period you chose, I'd say 18 is the easiest due to most of the obstacles being in their most neutered form and the lenient time limit. 25 had a harsh time limit and tough obstacles like the upgraded circle slider and dome steps.
For hardest agree with 19.
Rising: I'd say 29 for the easiest as the lumberjack climb is objectively harder than the rope ladder and I don't think any competitors had close call time outs in 29 due to it.
Agree with 28 for the hardest.
Morimoto era: I'd say 33 was the easiest due to having the longest time limit and easiest version of the fishbone and double pendulum.
Hardest I'd say would be 37, especially if weather is being taken into account.