Okay, rewind.
Why has no one mentioned the Mineral/Silver/Stone trade from Gwnet/Brycheniog/Dyfeg/Powys etc. to Pengwern? About four trips nets me the ponies I need for the wagon, and then from there I just send my wagon to a pengwern city with a strong detachment of heavy cav.
Easy money right there. And pengwern usually has salt for cheap, so those first couple trips you can essentially make bank.
Otherwise yeah just steal cattle, get the hides, dump in tannery, profit.
(Side Note: Ya know, that reminds me a lot of the gold trade in the west coast of Africa with the Ghana Empire. You know, gold for salt, salt for gold. Or the Egyptians and Phoenicians.)