Well you cannot do “band steering” with Mikrotik. This trick by dropping weak signal connections makes the client device reconnect to an AP. The client device chooses the AP and band again, mostly based on signal strength.
These are negative numbers, so the high number (-120) is a very weak signal.
-60 dBm is a strong enough signal for a very good connection. Things should work well at even lower values, like -73dBm or even -80 dBm.
The usual tactic to make client devices prefer the WLAN2 5GHz , is by lowering the TX power of WLAN1 by 7 dBm. The 5GHz antenna is two times smaller than the 2.4GHz antenna and therefor has a weaker received signal of about 6 dBm. So the client device prefers the 2.4GHz. if both are set to the same TX power.
The easiest and safest way to lower the TXpower of 2.4GHz with 7 dBm is adding 7 to the antenna gain setting of that WLAN wirelesss configuration setting.