‘To the Strongest’ by Simon Miller is a set of big battle rules where you can finish a game in an evening’s play. There are full PDF versions of the rules you can get from his website, https://bigredbatshop.co.uk/, for just £10.
Although aimed at Ancients it does have free lists for Early Medieval (i.e. Dark Ages), and I have seen games with a thousand figures a side being run quickly and easily.
The Ministry of Gentlemanly Warfare