Well what you’re probably looking to setup is 802.11r, but I think you’re still going to run into issues because of the proximity of where your routers are.

The issue you’re seeing is related to band shaping and signal-to-noise ratio. Your wifi client is actutthe thing that is supposed to be more smoothly handling the transition between access points with your current setup, but it may not work as expected without the signal for one or the other to be drastically worse than the other. 802.11r helps with that. Results are hit or miss though, so don’t go buying new equipment just to try it out.

If you had two OpenWRT devices though, I would just make a mesh and skip the above.