The "frowned upon"[1] warning is only about the second bridge, since that added wifi doesn't need to be into a bridge, as it is only used to access the router and not other devices on the network, as a "standalone" interface, it might be just fine.
And if you think about it, adding an "offbridge" interface to a bridge is a contradiction even lexically.
[1] more properly "stern look of disapproval" 
https://www.marriedtothesea.com/092110/stern-look-of-disapproval.gif
Haha! Yes, true, when I thought about it, if I am to get a look if disapproval, it should be off bridge, and thus avoiding an ironic disaster of locking myself out of my offbridge config when changing it to a bridge, and hosing my router entirely.
Disaster averted:
[me@MikroTik] > /interface wifi print detail where name=wifi-mgmt
Flags: M - master; D - dynamic; B - bound; X - disabled, I - inactive, R - running
0 B ;;; CST-MGMT
name="wifi-mgmt" l2mtu=1560 mac-address=xx arp-timeout=auto master-interface=wifi1
configuration.mode=ap .ssid="yy" .hide-ssid=yes
security=wifi-mgmt-sec
security.authentication-types=wpa2-psk,wpa3-psk
[me@MikroTik] > /interface bridge port print where interface=wifi-mgmt
[me@MikroTik] >
Hmmm.
You are really craving for that stern look, don't you?
What is "Hide SSID" for
L009 "can" be used as switch (I use it like that in my home lab).
And it seems S009 is coming (pure switch, 7x 2.5Gb ports and 1x 10Gb as well as SFP+)
Fine, I unhid it. Hopefully I am back to 1 stern gif.