Looking at your configuration it looks like you have VLANs 7 and 8 in use.
However, you also have two bridges. I don't know why this ever worked for you, but that's not the correct configuration. You should only have one bridge, with VLANs 7 and 8 added to that bridge.
Do you want an implicit VLAN 1 to not exist, so that the only VLANs that exist are VLANs 7 and 8? If so, which of these would you consider your "master" or most secure VLAN?
Yes, I have two VLANs - 7 and 8 (default VLAN1 is "converted" to 7). My main internal VLAN is VLAN7 (equivalent to default VLAN1) and guest VLAN is VLAN8 (only internet access).
So theoretically, I should have bridge (with PVID7 - my default PVID is not 1 but 7) and over that vlan8 with VLAN ID: 8 - like in updated config in attachment, right?
New configuration seems to work well with ROS 7.8. However, after updating to 7.10beta5 issue with connecting wireless client is back. Same behaviour with 7.9 and later.
15:28:20 system,critical,info ntp change time May/11/2023 15:27:54 => May/11/2023 15:28:20
15:28:22 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:28:48 wireless,info XX:XX:XX:XX:XX:XX@wifi2-host disconnected, connection lost, signal strength -66
15:28:52 wireless,info XX:XX:XX:XX:XX:XX@wifi2-host connected, signal strength -57
15:28:59 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:29:15 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:29:27 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:29:29 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.20 for YY:YY:YY:YY:YY:YY without success
15:29:40 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:29:58 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:30:11 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:30:15 wireless,info XX:XX:XX:XX:XX:XX@wifi2-host disconnected, connection lost, signal strength -57
15:30:20 wireless,info XX:XX:XX:XX:XX:XX@wifi2-host connected, signal strength -68
15:30:28 wireless,info YY:YY:YY:YY:YY:YY@wifi2-host disconnected, connection lost, signal strength -34
15:30:29 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:30:30 wireless,info YY:YY:YY:YY:YY:YY@wifi2-host connected, signal strength -35
15:30:45 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success
15:30:58 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.20 for YY:YY:YY:YY:YY:YY without success
15:31:02 dhcp,warning vlan8-host-dhcp offering lease 192.168.8.19 for XX:XX:XX:XX:XX:XX without success