Seamless WiFi roaming is mostly a client decision. The best you can do on AP is to:
a) optimize signal strength for coverage (stronger signals aren't always better in multi-AP deployment);
b) provide hints via 802.11k/v/r to help clients make, hopefully, better decisions;
c) forcefully drop and disassociate clients when signal is weak enough.
But if the client has bad WiFi implementation, there's nothing much you could do.
OpenWRT currently supports 802.11k/v/r, but optimizing coverage by adjusting signal strength and channels is left for experienced users to deal with manually. There is the are where some commercial offerings will do, but the result greatly varies. AFAIK there's no ideal system anyway coz physics is hard.
