What you're after, transparent wifi roaming, is actually mostly handled by the client; what you need is wifi access points that don't get in the way.

I don't have much experience with new OpenWRT supporting products, but the kicker is you only need one of them. If you have multiple routers, they will require some setup to play nice with each other. An "Access point" is just the wifi provider, can be hooked up to provide whatever the one router manages, and are generally cheaper than a router.

To that end, I'd suggest a single router, and multiple access points. I do this with Ubiquiti access points in my home, their PoE has been nice and they have been pretty "setup once and forget" for a few years now. I'm sure there are some other brands that'll do well; Ruckus and Mikrotik come to mind.