If there is any way you can run a few ethernet cables as a backbone, I think it's pretty simple to just setup additional OpenWRT access points on the same SSID to allow client roaming.
Or, with dual-radio routers, setup WDS between the routers using one radio per router and clients on the other, so traffic does not compete for the same channel. I am not sure whether you can still do roaming this way as I've never tried it. But I did use it in the past to brdige wired clients from a secondary router back to the main router over WDS.
Wood is not either. The inside walls here are 100mm wood(CLT) with gypsum on both sides and the signal drop through 1 wall is big. 2 walls almost kill the signal. I don't have any wifi reception outside the house at all (65m² is floor area) using 1 UI Wifi 6 LR on each floor (with the U6Pro it was even worse).
Or, with dual-radio routers, setup WDS between the routers using one radio per router and clients on the other, so traffic does not compete for the same channel. I am not sure whether you can still do roaming this way as I've never tried it. But I did use it in the past to brdige wired clients from a secondary router back to the main router over WDS.