I don't think you'll get lower performance for yours N and AC clients.
As far as I remember, the AX56U was always the cheapest Wi-Fi 6 model with Merlin support - I bought it for less than $100 equivalent (regular price, I see the AX58U at a price of around $110 equivalent). However, prices in your country may be different.
Higher MIMO Radio's generally penetrates walls better.. Every 2x2 unit I've tried including AX58U is worse in my environment vs 3x3 or even 4x4 units.. At least as a single point unit.
Wired backhaul will obviously yield better results as broadcast locations are more spread out. 2x2 is fine in this situation and is the reason most Wireless backhauls use a 4x4 radio to communicate between points with 2x2 client connectivity.
IE: 4x4 Node to Node + 2x2 + 2x2 for your 5G/2.4G connection. Wired backhaul works the same way, just avoids the need of node to node radio.
The lowest I see on SlickDeals (in the past 6 months) for new units is $120 for the 56U and $150 for the 58U -- both in USD.
From Asus:
56U
WiFi 4 (802.11n) : up to 433 Mbps
WiFi 5 (802.11ac) : up to 867 Mbps
58U
WiFi 4 (802.11n) : up to 300 Mbps
WiFi 5 (802.11ac) (1024QAM) : up to 1733 Mbps
Doesn't the above value cut in half if the client doesn't support 1024QAM?
68U
WiFi 4 (802.11n) : up to 450 Mbps
WiFi 5 (802.11ac) : up to 1625 Mbps
MAX QAM spec is irrelevant, especially at distance... You'll likely hover in 500-700 range at 25-30FT+ through walls with a 4x4 AX router. You have to test this yourself.
Most AC client's will only support a max transfer of 866.7.. Regardless of being capable of higher QAM... AX clients included. AC Nitro/Turbo QAM only works on niche clients like ASUS PCE 88 as an example.
The best single point compromise is likely a QCA based 8x8 AX89X, granted, I haven't had a chance to test it.. I'd prefer to pay under $300 for such an old design.. Netgear's version has gone as low as $225, but their firmware/SW is horrible in general
I'll say it again, your best option if you care about distance, updates and modern enhancements is to buy 2x 68U or 86U/S (id argue for AC86U/GT-AC2900 if significantly cheaper) and set connection sensitivity per client.
Obviously need to test which offers better performance/value. We cant help you there.