MT7621 - ISP speed change, messy configuration

Hi guys,
My current ISP will provide soon higher speeds (async sadly) and that got me thinking about my setup (xiaomi 3g v1). Current speeds are 200/20 and I was thinking about getting 500/50( I can buy R7800 for around 100euro, but maybe there is a way to do what I need cheaper)
My current network looks like this:

xiaomi 3g router + tenda mesh system( 0 configurations options - either bridge mode or router) + switch for my LAN devices.

Tenda was bought originally to get WiFi for my AndroidTV. The situation did change I had to share my network connection with my family(my router has a very poor wifi speed).
I tried to separate them from my network, but since Tenda does not have many options I've connected their Client AP to a guest AP and my TV to normal AP.
Using VLAN's I've separated the rest of Tenda mesh from "my devices" giving access just for the Internet + Plex so my TV can reach it.
As the current setting does "work" - but it's quite messy, and I have doubts my router will be able to handle 500/50(On stock OS it should but then I won't be able to separate them from my network). Is there a cheaper way to maintain the separation and get higher speeds from my ISP besides getting a new router like r7800?

I am using a mt7621 device with a 500/500 mbit connection just fine. As long as you don't want to shape at those speeds with SQM your router won't bottleneck your connection.