Multi-point WiFi configuration options

Question

I have a property which contains a significant amount of metal structure between the floors, but not between rooms on each floor.

Currently because of this to achieve a decent WiFi signal throughout the house I use three Sumvision SVW307R wireless routers, one on each floor. These are wired direct to the main home switch.

Initially I configured these on not overlapping frequency bands with the same SSiD.

With this configuration I found that as I moved through the house the devices, i.e. mobile phones; laptops etc. would not drop the signal and reconnect to the strongest signals reliably. Basically the device would hang on to a very weak signal to the point where connection speeds were unusable. This behaviour continued even though a perfectly good strong signal was available, albeit on a different frequency band. I also discovered the phones would not always choose the strongest signal even if I cycled the WiFi on the devices off and on again.

To resolve this issue I have now changed all the SSiDs so that I can choose the SSiD I wish. For example XXXX-GF;XXXX-1F;XXXX-2F for Ground Floor, 1st Floor and 2nd Floor respectively.

This way I can at least select the most powerful signal explicitly, but it's not a smooth solution, and I'd really like to have the WiFi configured so that all devices will correctly choose the most powerful signals and not mess about with low grade signals even though the device recognises it.

I understand that the propensity of a device to "Roam" to a new WiFi signal can be adjusted on some devices, but as not all devices support this tuning it is not a viable long-term solution.

Can any networking guru suggest the best way to resolve these issues, ideally with the existing hardware, although I'm up to replace the hardware if it's the only viable solution.

Many thanks in advance, D.