The nbg6817 is basically the same hardware as your r7800, so the same background applies:
While I would expect a (wired) throughput of around ~550-650 MBit/s (well, you don't say anything about your WAN configuration, if you need PPPoE, ~300 MBit/s might be reasonable), it won't do anything close to 1 GBit/s.
The WiFi performance is a bit harder to judge, without knowing a lot more about your environment (congested environments with many concurrent neighbouring APs will produce more interference -the airtime simply being exhausted- than rural areas where you might not even see your neighbours' houses), nor do you tell anything about the test distance or the clients used for these tests. Most clients (notebooks, phones) will be 2x2 at best, providing you with a (theoretical, not achievable in practice) link rate of 866 MBit/s - under these circumstances, the best you will get out of it in practice will be around ~300-350 (400, if you're very lucky, but neither would ~250 be anything to complain about with lower end clients) MBit/s. 75 MBit/s sounds a bit as if your client would opt to use the 2.4 GHz band, for which there may be several reasons (keep in mind that 5 GHz will take longer to come/ get found by clients, clients may be sticky, etc. pp.).
Always check wired(-only) performance first, if you're in doubt about your router's performance or potential issues with your WAN connection.
At least for the initial WiFi performance testing, avoid complex (multi-vAP) configurations, while those should not cause this amount of throughput loss, there will be some - and the additional interfaces will also delay the interface coming up as well (sticky clients).
I would strongly suggest a USB3 ethernet card for the RPi4, to act as dedicated WAN interface, as that massively eases the configuration of your setup. Technically the r7800 can act as L2-managed 5-port switch as well (at no additional performance cost), but (dedicated-) switches are often more convenient. Personally, I prefer (smart-)managed switches these days (and there are quite a few OpenWrt supported ones, even rather cheap on the second hand markets), but technically you can rope in the r7800 for this task - with unmanaged switches behind the VLAN distribution done on the r7800.
From personal experience, I'd also suggest to look for devices like these (the ones raised in the later parts of that thread, not the first AMD Jaguar based ones):
The r7800 still is a very decent 802.11ac/wave2 AP (although 802.11ax APs can achieve 700-800 MBit/s (at 2x2) in comparison).
 USB3 required (not USB2); realtek r815x based ones (starting around ~15 EUR/ USD) seem to be preferred by RPi users, as they tend to be faster and more reliable than their ASIX AX8817X competition.
 the RPi4 can easily cope with these WAN speeds, they are just hard to get these days and overpriced.
[2} and this is also be required to really get 1 GBit/s full duplex, rather than 'just' half-duplex a "one-armed router on a stick" can provide.