To be clear, OpenWrt doesn't have anything to do with roaming. Unless you are running 802.11r/k/v, the AP actually has literally nothing to do with it, ever, regardless of the firmware on the AP. It is a client side process.
So, if your results with OpenWrt are not as good as the stock firmware, that suggests one of two things:
- Driver issues/limitations
and/or - Configuration parameters not properly optimized.
This is generally a bad idea for roaming. Roaming works best when the channels are different.