I have two parallel games going, a storyline play through and a raider sandbox play through. I've stopped the latter till the Vegr, Denmark AI gets fixed so they'll attack across the sea. You really need them to attack in late mid game so you can hop into to lord vs lord fights for trophy scumming. In the sandbox play through I became a vassal and did a lot of trophy farming kiting Alban armies from my village to various lords (AI will attack across sea to loot your village).
There's a few things you can do to level up. Villages do have a small militia, usually about 30 or so villagers. If you raid a village with an army of 34 or so men (or less) they'll fight you, much more than that they'll run off and you'll get the option to burn and loot. So just attack with a small force, fight the villagers and after you win leave, don't take the burn and loot option, unless of course you're sure there are no enemy armies around. I did most of my leveling signing up as a merc to Vegr and repeatedly doing hit and runs on the Alban and Aileach villages. A 39 crew boat is perfect for that. If you have 10 or 15 gestr or better warrior archers, 10 or so foot and the rest companions and you then you shouldn't take any casualties. Companions up front to engage, archers to do most of the killing and the 10 or warriors just to scare off the occasional semi small bandit bands if you aren't ready for them. Most bandits will run from a 30+ man army, they'll engage a 20+ one and that can get dicey if you're hurt. That way you can decide when to fight.
I'd strongly caution against raiding monestaries, that really pisses them off. Once in a while for food is ok but I'd only do it in a pinch. Later when you have a good strong 150 man army and a refuge feel free to raid monestaries, gives the lords a reason to attack you and you won't lose reputation honor when you kill them. Be warned though that it makes starting a kingdom tough.
To be a good raider you really need a larger inventory and skills designed around fast movement and spotting. I wouldn't worry too much about trader skill, a lot of your money comes from selling slaves.
The two three biggest tweaks I'd see are Vegr/Denmark AI, a bit more variety in bandits (huge gap between bandits and vikings) and freelancer. Next play through I'm going to try to play nice with the Swedish farming fleet and see if we can be friends.