If it was working with the previous version - it's not the hardware itself. It should be able to do at least 100-200Mbps. You should check and compare the memory usage as well. On the firewall page there should be some checkboxes, which may lead to some improvement if not trying to use QoS. You may compare the performance over the 2.4GHz band - if it's working better, there could be dust inside, causing signall loss over the higher frequency band.
Do I understand correctly? Am I doing a proper summary:
On NETGEAR WAX206 you have little or no proprietary BLOBS, whereas on Xiaomi AX3600 you are very dependent on proprietary BLOBS (not proper open source OpenWrt possible)?
R7800 can work with 160MHz channel so it should make more on wider channel, but it has only one gig ports - so it's plenty of power with nss hw firmware. Compared to dynalink is similiar, but with 2.5gig port should be even faster, so all basicly depends on question what performance You want . Also if the clients suport ax or even 160MHz channels.