Load Balancing with MWAN3 on a GLI 750s Router using Open WRT

Hi there,
I have a GLI AR750S (slate) router.
I want to set up load balancing and require MWAN3 setting up on openWRT.

It is a little above me to setup, but I do want understand it enough to adjust some parameters and fault find if needed.

This is the document to set it up
https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3

This is some additional code someone else made which is even more relevant.
https://gist.github.com/braian87b/97a186b2e11b5aa438d8fd17de0eab20

The GLI is used as I travel between locations, connecting to existing routers to create my own internal network for my devices.

WIFI - will always be connected as a WAN source
ETH - will be connected if the GLI is physically close enough to the local router
PHONE TETHER - I will connect my phone to the router via USB (or can do wifi if easier?) as a 3rd source for WAN

I'd love to load balance upload/download, but I've read that this is not possible.
IE if I have one computer connected to the GLI, on a zoom call for example, can I make use of BOTH WAN connections (wifi and tether) to provide greater connection speeds? I've read this is not possible, but if my reading is wrong and it is, this would be ideal.

If not, then I would at least like to enjoy smooth failover. The internet connections where I am drop out for 10 seconds every 2 minutes or so. So i'd like to be able to be on a zoom call and at least failover smoothly to the 4g tether from a smart phone when it does drop out and then smoothly switch back to local wifi when it comes back on.

I hope this is enough info,

I look forward to a proposal

Kind regards
Daniel