SimBrief already optimizes the altitude selection actually. The dispatch remarks will normally say “Planned optimum altitude” when it chooses something lower due to winds. Right now it simply targets the best fuel vs time ratio, rather than a purely min fuel or min time strategy.
Coming back to your flight specifically, I don’t believe it was actually worth it to cruise lower. For most transport category jets, it’s extremely rare for an altitude such as FL280 to be more efficient than FL380. There would have to be much less wind down there (at least 50-70 kts less) for it to actually be worth it from a fuel perspective.
On your flight, the headwinds were only about 20kts weaker at FL280, and they were actually 1kt stronger at FL320. Nowhere near enough for the system to choose those altitudes instead of FL380.
For reference, I have pasted an excerpt of your flight plans below (click to zoom):
The first, at FL380, shows a trip burn of 7839, an air time of 3:21, an average headwind of 71 kts, and an average fuel flow of 2340. The second, at FL280, showed burn 9101, air time of 3:12, headwind of 57 kts, and average flow of 2832.
FL280 was indeed faster, but it burned 16% more. In most cases an airline would not consider this to be a worthwhile tradeoff, unless they absolutely had to get there as fast as possible at any cost.
You might have been looking at the block fuel number, which is indeed a bit lower on the FL280 flight. But this was due to a difference in the ETOPS scenario which necessitated extra ETOPS fuel. I’m not sure why that was (maybe an alternate fell below minimums on your later flight or something), but it is unrelated to your altitude optimization question.
Hope this helps,