Desired outcome - Allow any Eth port on my HAPAC3 to be plugged into an existing network and have the existing DHCP range be passed-through to whatever else I plug into the other Eth ports / connect to the Wifi of the HAPAC3.

On more basic routers like Asus / Tp-link etc, this is accomplished by disabling the routers DHCP server, and then plugging it into the existing network.

I've attempted to set this up myself, but I must be missing some config / understanding.

# jan/02/1970 00:54:11 by RouterOS 6.47.8
# software id = A06G-RL1H
#
# model = RBD53iG-5HacD2HnD
# serial number = ***REMOVED****
/interface bridge
add name=bridge1
/interface wireless
set [ find default-name=wlan1 ] disabled=no ssid=MikroTik
set [ find default-name=wlan2 ] disabled=no ssid=MikroTik
/interface wireless security-profiles
set [ find default=yes ] authentication-types=wpa2-psk,wpa2-eap mode=dynamic-keys supplicant-identity=MikroTik wpa2-pre-shared-key="\***REMOVED****"
add authentication-types=wpa2-psk,wpa2-eap mode=dynamic-keys name=profile1-2.4G supplicant-identity="" wpa2-pre-shared-key="\***REMOVED****"
add authentication-types=wpa2-psk,wpa2-eap mode=dynamic-keys name=profile2-5Ghz supplicant-identity="" wpa2-pre-shared-key="\***REMOVED****"
/interface wireless
add disabled=no keepalive-frames=disabled mac-address=***REMOVED**** master-interface=wlan1 multicast-buffering=disabled name=wlan3-2.4 security-profile=profile1-2.4G ssid=2.4G wds-cost-range=0 wds-default-cost=0 wps-mode=disabled
add disabled=no keepalive-frames=disabled mac-address=***REMOVED**** master-interface=wlan2 multicast-buffering=disabled name=wlan4-5G security-profile=profile2-5Ghz ssid=5G wds-cost-range=0 wds-default-cost=0 wps-mode=disabled
/interface bridge port
add bridge=bridge1 interface=all
[admin@MikroTik] >

Your try seems mostly fine. You can try to manually add interfaces one by one to bridge instead of using interface=all, this construct is not used often and could feature some hidden bug.

Thank you @mkx.

Removing all i-faces and adding Eth1-5 + WLAN1-2 gave me the result I want. Seem's like adding the bridge, breaks the config. I think this is a lack of understanding on my side, and not a bug.

From default...
Remove firewall filter and nat
Remove DHCP server
Remove IP address from bridge.
Move DHCP client to bridge.
Add ether-1 to bridge.

Then you should have a 5 port dual band WAP to configure.

Using the Wifi WAVE2 driver gave me a much better client serving radio... But it broke a few things in my ACLs.

So basic WAP... Use the WAVE2 drivers. The ACv2 works far better than the older standard.