Some really solid observations there waggamick.

As for firmers and drifters, I've read an article that says more drifters win overall than firmers [what is naturally assumed]. I can't confirm whether it's true or not [?]

4 and 5yr. olds are the best betting targets overall. They are in their prime, have established form and are not yet into the nagging small injuries and even getting sick of the whole game that often starts to appear in 6yr. olds.
Punters of course are the last to know when these older horses are in good all-over condition and 'set' for a race [like older athletes].

We start to see the word 'freshened' very often with horses over 5yrs. old. What that really means is that the horse was bugged-up from a run or had some injury or other health problem attended too [best to avoid these 'freshened' horses]. Generally, 6yr. olds are on their way down and 4yr. olds on their way up. Worth looking at the 4yr. olds in races against older horses, especially over sprint distances 1000m to 1600m.

Following 4 and 5yr.olds from good trainers until a 1st. prep. win only has a lot going for it as the odds stuff up after that. Things can be narrowed down by becoming familiar with the way trainers prep. most of their horses. Mick Price for instance presents horses forward in condt. so the early starts are the ones to be on with his horses.

Different tracks suit different trainers [and jockies] too. For instance, Hayes doesn't make the top 10 at the Valley but his runner's odds will be down regardless:

Bridget Flynn 25.0% [SR]
Price, M G 22.0%
Edwards, J W 22.0%
O'brien, D T 19.0%
Maher, P F 18.4%
Tom Hughes 18.4%
Freedman, D L 17.0%
Fox, Ms K 16.0%
Martin, T R 14.0%
Symons, J G

It's worth checking out who does well where. Punters could do a lot worse than following some good trainers. Some of the highest SR's come from trainers most punters have barely heard off and their runner's odds are not so affected like the big name trainers. They don't present a lot of starters but a punter wouldn't lose out by grabbing a fist full of the better but less know trainers with high SR's and following them and their generally better odds.