I would set up both the SXTsq and hap AC lite as Routers.
Reduces broadcast over the wireless link if nothing else.
Also, if you lose the link, you still have a local LAN (and likely can
tether your phone to the hap AC lite)
Change the LAN IP address range and DHCP range on the hap AC lite from the default 192.168.88.0/24 to something else. (Can also change the LAN address on the SXTsq as well)
This can be done using quickset, but take care, see the longer explanation of rule 4 in the link below.
https://forum.mikrotik.com/t/the-twelve-rules-of-mikrotik-club/182164
Yes, you can use ether 5.
I would use winbox, maybe webfig for the following.
Remove ether 5 from the bridge ports list and under interface / list add ether5 to the WAN interface list.
Move the default dhcp client from ether1 to ether5
Add ether1 to the bridge ports list (ether1 is now a LAN port)
I would then go to /ip services and disable all services you don't need.
I would normally disable all (disable-able) services except winbox, on both the hap and the SXTsq.
(You might want web access)
Turn internet detect off.
(Interface tab of Interfaces window, button at top)
or via cli
/interface detect-internet
set detect-interface-list=none internet-interface-list=none lan-interface-list=
none request-interval=2m wan-interface-list=none