I'm glad the script is working for you.
I used a local bridge for simplicity, it's hard to guess exactly what you have on your side...some devices don't have switch options, and other configs may have distracted or confused the issue. Ports off my router are trunks, someone else may be using them as access ports in to various VLAN. Configure them to suit your specific needs and performance objectives.
Accepting established,related on the WAN allows traffic you've initiated to pass through the router. Note the input and forward drops on the WAN interfaces. Again, a very simple config with basic WAN related protection. Adding to your VPN only, you almost certainly are going to want things you request to be allowed through the firewall (DNS, HTTP, HTTPS). Should you not allow established,related you won't be browsing the web.
There are a few schools on how to handle internal interfaces, if you choose to be restrictive, then yes you would need allow rules between your internal networks. The sample I gave assumed a flat topology, with no need for restriction.
To reduce bandwidth hogs you can take a few different approaches. The most simple would be to outright block sites or systems of concern. You could also look at queues, that will start to get things complicated. The below rules will slow everything on LTE down to 512k, you can be more precise or creative with marking traffic.
/ip firewall mangle
add action=mark-packet chain=forward in-interface=LTE-WAN new-packet-mark=LTE-LIMIT passthrough=no
/queue simple
add max-limit=512k/512k name=LTE-IN-LIMIT packet-marks=LTE-LIMIT queue=ethernet-default/ethernet-default target=""