Multiple VLANs behind RED10/15 including GUEST-network

Hello
for branch offices we are using accesspoints that do local switching for each SSID to a separate VLAN.
Branches have a L3 switch installed that routes the local VLANs via the RED-interface of the HQ-UTM

HQ UTM has a static route pointing the remote VLANs via the L3 switch.
L3 switch does DHCP relay to HQ-DHCP servers (and for that requires an IP address in each VLAN)

So far so good :)

Trouble now is that we now need to deploy a GUEST-SSID in the remote offices with the following requirements:

• no local routing between corporate and GUEST-VLANs
• DHCP for GUEST-VLAN done by HQ-UTM

Problem is: L3 switch requires IP for DHCP relay but afaik automatically routes between its local networks once an IP is assigned to a VLAN therefore requirement one is broken

Any ideas how to solve this issue?

This affects multiple remote GUEST-SSIDs

Thanks for advice