Two ideas to discuss, possibly . . .
Firstly, as a house rule, cavalry units cannot dig their own trenches. I know cavalrymen were able to use shovels, but their main role was to exploit gaps and disrupt enemy supply lines. They can, however, move into trenchworks already dug by infantry. I quite like this house rule as it makes me think harder about how I use my cavalry.
Secondly, should cavalry units be able to Reform? Or if they are destroyed should they cost full price to replace? Cavalry seem to feature less and less in the literature as the war progresses, although this may be a Western Front development. Any thoughts on this?
Historical part -
Cavalry was maintained extensively by the UK, tens of thousands of Horses and men by Haig in the rear for 4 years basically played emergency firefighters waiting for the inevitable charge to victory.
The Germans quickly switched all their cavalry to the east, where it was priceless to patrol, scout and keep the peace in the countryside. Vast spaces of Poland and Byelorussia and Ukraine had the German and Austrian/Hungarian cavalry (later supported by Polish cavalry) against Russian cavalry & Cossacks in charges and counter-charges often against machine guns (albeit with huge losses to Cossacks and Russian cavalry).
Think of cavalry as mounted infantry without heavy weapons.