New opnSense user : my first multiWAN [help requested]

[toxic]

Hello.
My current situation : I live in a remote place, with poor DSL service and good 4G-LTE coverage, so I have 2 ISP subscriptions with a router-box provided by each : DSL is slow but volume is unlimited, 4G is fast but volume is expensive...

 I currently have a rather complicated setup that we won't go into details but I will reconfigure everything when my new alix 2d13 arrives and meanwhile I have a VM running opnsense to allow me to do some tests...
Attached is my future network setup as far as I can imagine it.

My main question concerns how to handle the multi-gateway.
As of today, my piHole DHCP server is setup to assign permanent leases to all my devices, and per MAC address I can define which gateway the DHCP will indicate to the host. This allows me to give the IP of the ISP's 4G box to my laptop or phones (nice fast internet for these devices), and give the IP of the DSL-box as gateway for my smartTV (slower internet but no volume issue since unlimited on DSL).
Currently this is on a per-host-macaddress basis and works fine since all the device resides on a single network with the same subnet mask.

When I switch to my new opnSense router, I would like your advice on how to deal with this. Ideally I would :
- get rid of piHole as my DHCP and use opnSense instead for that
- possibly switch from 4G or DSL based on the remote service I'm using (netflix and youtube through the DSL, google and facebook through the 4G for example) transparently for each host
- have a failover (if 4G is down or slow then fallback to DSL and other way around also) transparently for all hosts
all that together of course
oh, and of course, all these smart-home things I have like my TV can only use DHCP... So beside my raspberryPi and my NAS server I need/like to have every device just auto-configured by DHCP.

Thing is : I don't know how to do that with opnsense... Cloud you please help me in setting up opnsense for that ? Idealy my undestanding is that I would move DHCP to opnSense and use opnSense as defautl gateway for eveyone and setup some logic inside to define which upsetram gateway to use.

I'd also like to know if I can revert to something similar to what I have now for the multiGateway (one by host), but I did not see how I could setup opnSense to give different gateways in the DHCP offer based on the host. I use set:tag0 or set:tag1 in my dns-hosts definitions on my piHole for that at the moment, how could I do this with opnSense ?

Also, as a last thought, I'd like you to confirm that I correctly understood something :
both ISP provided router have by default the same 192.168.1.1/24 IP address and provide DCHP leases in the same range. If I leave it like that and plug them both on WAN1 and WAN2 on opnSense, my understanding is that opnSense will be unable to distinguish the gateway to use base on interface only and won't be able to do failover or other FW rules, so I will need to change the settings of at least one box so that they don't have the same IP.

Thanks a lot for your time reading and thanks in advance for your kind help !