I am aiming into East Germany region.
Cavalry was very rare in western slavic army. Most of the units should probably be similar to Saxons in Dux, although not so heavily armed.
Western slavs used terrain heavily. And for most part slavs fought on foot, with cavary only as kings retainers.
Problem is that most of the tribes didnt had hereditary ruling family but were tribal quasi democracies. That did not help with westernization of warfare.
So Main force should be standard warriors with shields but without armour apart from helmets. with elite armored druzinha and armored foot northern (Danish, Wiking, Jomsviking, Polish, Western Pommeranian) mercenaries or allies. Cavalry only as single unit and later in the campaign = lot of archers/slingers.
I agree that balance could be a problem and maybe should be overcame by heavy use of terrain. Most successful Saxon raids were in winter, when marshes and rivers were frozen.
because when in bad situation Slavic forces often would just disappear in terrain and refuse fight.
I suppose it should be trial by error in playtest.
Problem with the Raider rules is that Slavs were mostly raided part not raiders, so I don't know if Raider forces could work (I didn't got supplement either - as I was never tempted into Britain really. I got the Dux at sale as an afterthought and I was surprised how cool its campaign system is (although I am yet to play aa actual game - I am just easily bought by cool campaign systems and pretty maps and the general idea of fall of the old order
Generally I would be very shy with cavalry unless you want to play late Lithuania (ie more southern) But north Lithuania was not very supportive to cavalry use. Very heavily forested area. Only later in XIII century when Teutonic order destroyed Jaćwięgs Lithuania proper was targeted but under Mendog it was proper country and not tribal federation.
Northern tribes of Todays Poland and Lithuania were not Wends (Wends were Slavic) but Baltic tribes but I would guess that terrain use and light to medium infantry and tactics would be quite similar.
Although we know that Jaćwięgs herded Tarpan horses - very small forest horses - we don't know about their cavalry.
There are no archaeological findings of Prussians armor and also in written sources only defensive element they used was shield. Main armament consisted of spear, shield, sword or axe, there were throwing clubs, slings and there are know instance of them just throwing rocks.
In XIII century proper enemy for Samogitians /Curonians would be Fratres militiae Christi de Livonia and after disastrous 1236 - Teutonic Order.
Key of Baltic victory in 1236 was terrain unsuitable for heavy cavalry.
But at that late period tribal forces are just too obsolete to fight in other circumstances.
More interesting enemy would be Lithuanians under Mendog in 1240+