In terms of HW specifications, Mi 3G router is a downgrade from DIR-865L which is a decent router from Starhub even though it is old. DIR-868L is a minor upgrade for DIR-865L and can be bought from Carousell at S$20-30.
Mi 3G : AC1200, 2 Gigabit Ethernet ports only.
DIR-865L: AC1750, 4 Gigabit Ethernet ports
Do you have any problems with the DIR-865L? My one is still working well with Starhub 500Mbps broadband. Not intend to change yet as the tenants have no complaints about its performance. Coverage is decent too for the 118sqm flat.
Not so sure about Mi Router Pro's range and coverage, but at least the HW specifications are better than DIR-865 with AC2600 and 1 USB 3.0 port, three Gigabit Ethernet ports only but hopefully that is fine.
If you buy the Mi Router Pro, you can still set up the DIR-865 in AP mode or repeater mode in case you need better coverage in some locations.