Simple VLAN between a chateau and 1 port from CSS326

Hey all any help is a blessing, i am kind of new to vlans on mikrotik

I have a chateau giving 5g internet through eth1 on a CSS switch (port1) . I want to connect an secondary ISP router on CSS Switch (port4) so that i can have a backup plan for when 5g is not available. The problem is that i have only 1 cable running from CSS to the Chateau, and both routers have DHCP ( i cannot turn off dhcp from ISPs router). So i think i will have to go the VLAN way.

My thought is that since CSS forwards all VLANS by default, and from what i read all untagged traffic are vlan id 1, it will simply continue to work as it used to do, but i will also have a vlan with the secondary ISP comming into the chateau for failback through the same port.

Correct me if i am wrong, but i am thinking of the following.

Create a VLAN (eg. VLAN200) on Chateau and assign it to eth1,

Go into CSS and strict the port4 so that it only accept VLAN200 tagged packets .

Add a DHCP Client to the Chateau on the Vlan interface

Plug the secondary ISP router to the port4 of CSS

change the distance of the auto-created route of that DHCP client to something bigger than the distance of the route of the LTE interface.

And i am done?