All of your substance came from your very last sentence...
I'm unsure of what you're referring to with the side quests, because ultimately, in the RPGs, the game is full of "Side quests"... but background storylines are once-in-a-game... collected as "events", and are prized by dedicated players of the series. So those events can hardly be labelled repetitive side-quests.
Of all the RTK games, RTK VII's battle system can actually be praised. Whereas I've quit RTKXI long ago, RTKVII still brings me pleasure. RTKX is simple. RTKVIII makes me cringe. I guess I should summarize what I have to say just like you did. RTKVII has an infinite replay-value. The rest of the series does not.
Side Quests, as in things that are not related to the main game. I'm not talking about events, I'm talking about the elements in RTK X where you go find a sword smith, or bandits, or whatever.
I am not talking about background story lines at all
RTK VII's battle system is weak, but it only appears to be a good system. It really isn't. It even allowed you to attack somewhere with your army, and then defend with that same army in a different city on the same turn.
Also, I'm not even talking about XI at all
. I mentioned IX (9) and VI (6). Which are the two best examples of the entire series. Each of the games has a different thing going for it though. 7 has the best atmosphere for example (the music and art are absolutely wonderful). But as a game it is weak.