I'm using Pi-Hole on an Ubiquiti router together with WireGuard and DNSSEC. My Synology NAS is backup (with regards to the DNS-based Pi-Hole blocking) taking the adblocking load off the router (there's no redundancy for WireGuard endpoint though). I don't (need) to use a RPi anymore. It works extremely well for me, and all my clients also get to connect to Nextcloud running on the Synology. My setup does far more tha…
It's interesting you've found the WireGuard experience to be "seamless". I have a WireGuard VPN at home and experimented with always-on, on my Android phone. Unfortunately, my provider (EE, UK) throttles UDP traffic something rotten, and my normally great experience with 50/50Mb+ is severly limited to between 0 and 10 Mb making my phone almost unusable by normal standards. Does your LTE provider not throttle this way…
That being said, have you attempted to discuss the issue with them? Have you considered a non-default UDP port? Also, did you compare the usage with OpenVPN? I ran OpenVPN before, the roaming, network speed, and latency is quite terrible.