Do you want fries with that?
On a more serious note:
First of all, there is no DECT support in linux at all (not even a DECT subsystem), so you can scratch that from your list (needs an external solution). The same story goes for the rest of your requirements, yes, you can achieve that - but not within a single device and probably not quite within your budget.
With a >>1 GBit/s WAN requirement, you effectively end up with x86_64 (and not quite the lowest end devices either) as a router - especially if you want to offer that on the LAN side as well. Based on the technical specs alone, the ODROID H2+ would give you two 2.5GBASE-T ethernet ports - if it can actually keep up with routing that under full load would be another question… Depending on your requirements, a higher end x86_64 system with enough ethernet ports (2.5GBASE-T and/ or SFP+) might be in order. The former would fit into your budget, the later probably won't.
In order to service your LAN requirements of 4 ports, you'd need a smart-managed (L2) ethernet switch on top. The cheaper option would be to go with a 1 GBit/s model with sufficient ports (e.g. ZyXEL GS1900-8), but that would require a third ethernet port on the router (so you retain one LAN 2.5GBASE-T port), which is slightly problematic on the ODROID H2+. These start around 100 EUR, some even supported by OpenWrt.
There is no way to reasonably or cost-effectively add wifi6 support to x86_64 at this point (200+250 USD for cards with ~half a year lead time), so the only option is to add a dedicated wifi6 AP - ideally one with OpenWrt support or at least a chance of OpenWrt support in the future (basically (mt7621a || mt7622)+(mt7915e || mt7921e) or ipq807x; ipq60xx or ipq50xx in the more distant future). These start around 75-110 EUR, but can go way beyond that towards the higher end (mesh…). Obviously these estimates are per AP, for mesh you'd have to multiply…
As mentioned before, DECT is not possible with OpenWrt supported devices. You will need dedicated, proprietary hardware for this - Grandstream, Cisco or Gigaset (N510 IP Pro) would offer options for this, using an entry-level AVM Fritz!Box (in IPoE mode, behind your OpenWrt router) with DECT support and re-using existing CAT-iq 2.0 handsets might be more budget friendly. Either way, you're looking at just around 100 EUR for this.
I guess you can do the math on those budget requirements yourself.
If you don't actually get >1 GBit/s WAN speed from your ISP right now, it does make sense to skip this requirement for the time being and to wait for the really highend stuff to become cheaper (even ~2 years might make a substantial difference in terms of prices, availability and OpenWrt support here). 1 GBit/s WAN speed can be dealt with just about any (contemporary) x86_64 device, the RPi4, maybe The NanoPi r4s or Marvel ARMADA based devices (what all of those have in common, is being wired-only). Likewise the prices for >>1 GBit/s ethernet switches are still rather unreasonable for home uses, a used (even OpenWrt supported-) rtl838x smart-managed switch might make more sense for the time being.
If your current WAN speed requirements are below ~500 MBit/s, many more doors open right away - here it it possible to get 'everything' (except DECT) in a single OpenWrt supported device (as long as you don't expect SQM as well or VPN at top speed) and would have a much better chance to stay close to your budget range.