I originally had the intent to train for every single level, in order to max out skill levels, but to my surprise, when I leveled up I had 3 levels accumulated, so now I've lost the opportunity to train for two of those levels.

I got this idea from playing Oblivion (which I never played a whole lot of) - is it overkill / absurd to squeeze in skill training for every single level from level 5 on? Or is this a great way to really make the most of every level gain?

Your call. Depends if you want get higher quicker or not.

No matter what skill you gain or when you gain it, it will contribute to the next level up or the one after if you've already reached it. No cheating the system here.

I find that its almost impossible to train via a trainer in between every level in fact sometimes the training itself levels you up
That and the cost of training reaches a point where getting the cash to train causes you to multi level up before you can afford it (pre level 10 especially)

The prices and speed you level up seems to begin to even out at about level 10+

Im on my 3rd character all level 50+ except my new one who is level 46 atm all played on master difficulty with time set at 5min in game for 1min real time with as little fast travel as possible

On all of the chars I have made I used trainers mainly after level 10-20 because the first 10 or more levels I get in about a day or two in game making daggers etc