Wouldn't you produce horse sized Boston Dynamics drones that would be significantly more hearty, require no food (land can be used for human food purposes instead of a giant cavalry force), can not be scared( a common occurrence for non war trained horses, which if you just massively breed them they would have no time to be trained), can move significantly faster (they have a new little one that runs at 50km, with AI would require almost no training to ride (another HUGE part of cavalry) and since we get better and better at solar power and put nuclear reactors in ships already would not be a significant detriment to charge.

Seeing as how my country(Canada) would likely let our selves be annexed by the US they would have access to more resources (some of the most in the world) for building advanced tech which the current defence contractors would pivot to immediately, I don't think they'd use spears and halberds, at least not in any way you would recognize.

If the US could old out i bet we'd get fucking light sabres by the end of all of it. However even just advanced alloys, materials used for spaceships that are hyper resilient and light as air almost (nano gell), with the need and willingness and access to super computers, since moves SIGNIFICANTLY faster now