That's not at all how I remember it Gollum. Viking era spies are not hard to tech up to IIRC, and I don't see why you couldn't get them before you reached Wessex, unless you just wanted to spend your money on something else. Viking era provinces tend to be rebellious until you've held them a while, so spies are handy, but not essential of course.

I did not find spies as useful in early/high/late for improving loyalty really. Peasants are available everywhere there's a fort, and so cheap that the upkeep doesn't matter too much. Just find something to do with them after they are not needed in their current province, such as garrisoning a different province. That way you do not have to train too many. Since peasants offer a reasonable alternative to spies, I would use peasants so I could put other things in my castles' build queque, like teched up troops that neither spies nor peasants can replace. Not that this is an option once I started playing without peasants, but when you've got a mod, it all depends on how its modded.

Substituting peasants for spies is less attractive in Viking era, because peasants require their own special building that ties up your building queue anyway, so you might as well go all the way and tech up to a spy building. Also, a spy in Jutland is a handy way to keep the Vikings out of your hair, though not essential of course.