At the moment I owe all the settlements bar 7 or so; I have been supporting a number of expensive full stack armies so and extra couple shouldn’t make any difference.

Are there any spearmen that are particularly good against heavy cav?

Unfortunately the FM army is literally right next to the settlement (so that if I attack one then the other army arrives as reinforcements). The other issue is that if I attack (but fail to kill) the FM then in a couple of turns their bodyguards go back up to full strength.