Have you installed the required 120ohm termination resistors specifically needed for long runs of RS485 wiring? Only one on each end, and none on drops in between? By design it is very robust, using twisted pair for noise and interference tolerance. If the termination resistors are omitted there is ringing and poor transmission results.
You may find this may be far more reliable and robust than any wireless solution.
RS485 is widely used in industry specifically as it is noise tolerant and can go for very long distanced over a kilometre or 4000 feet, quote a longer distance than WiFi could possibly reach. It has been around for many years and combined with the MODBUS protocol is the preferred solution for robust industrial control.
Yes, the interface is serial, so should be transparent to any BMS controllers whether wired or wireless.
Prudent wiring practices, keeping parallel runs of electrical and control cables separate, or using shielding should take care of cross interference. You may find interference on radio signal may be an issue with your proposed solution, not just at installation time, but later as the building becomes fully utilised.
Cat 6e cable is cheap and the shielding should take care of any interference as long as the termination is done to balance the differential signalling. Only earth the shield on one end, another trap for new players.