One thing I would say, is try not to be scared of diving into traditional RTS gameplay. You are already competing with some of the best RTS games, whether you like it or not.
I don't think you have to go fully into traditional RTS movement though. Something as simple as queuing construction before I have money for it would solve the problem. Another idea, is offering options between different paths for your units to take. This could open up a whole new layer of strategy while keeping the same automated feeling it has now.
And for multiplayer, I don't mean you should not do it of course. I personally think that a really good single player AI should instead be your main focus though. This way you can draw in players with the single player experience, which could help boost your player count for multiplayer more in the long run. I think updating it is a great start, and maybe adding some random strategies that it will pick from.
For the buggy spam, I do think it's just a player feedback problem. It's not clear that I can ignore the spam of buggies. Adding some kind of turret that I can build to hard counter buggy spam would make it feel much better.
I just tried playing again and avoided using buggies, only to get absolutely crushed. Not sure what I'm missing but it still seems like you need to play perfectly to counter the AIs spam of buggies, or just keep spamming them yourself and try to sneak in some economy if you can.