If you actually want an update relative to the mt7621at+mt7603en+mt7612en based mir3g, those budgetary constraints are going to be a problem...
Maybe you can get mt7621at+mt7615n+mt7615n for that price, but it's not going to do VPN or SQM at anything close to 1 GBit/s (not even a third of that), nor would it offer wifi6.
While mt7621at+mt7905e+mt7915e would give you wifi6, the SOC remains the same, and with that the performance limits (further stressed by the CPU cycles going into the task of keeping the faster wireless happy).
The rt3200 would now bring you a faster SOC (mt7622bv) with wifi6 wireless, but SQM/ VPN won't reach anything close to 1 GBit/s either (around half of that for SQM, I don't have figures for VPN usage on this SOC - and the type of VPN also has a lot of impact of the performance). The Xiaomi ax3200 might meet your budget, but support for that depends on the exact hardware revision and/ or (a kind of fuzzy) manufacturing date…
ipq807x would give you wifi6e and a lot of CPU performance at good prices (pretty much spot on for the upper end of your budget, at least for the entry level models), but routing is still limited to to around 600 MBit/s - and ipq807x support as a whole hasn't been merged yet.
Routing at 1 GBit/s wirespeed already requires a top-end router, which usually comes for top end money. Adding SQM and/ or VPN on top, with an expectation to meet those figures even more, leaving you effectively with x86_64 only (modern core i5 and faster), plus external WiFi APs.