Even on Ethernet it's around the same speeds.
I think that maybe your APE6 is having HW issues.
If it was just wireless, I would say maybe interference and splitting the 2.4 and 5Ghz bands, and moving everything you can to 5Ghz.
Four years ago, I started having issues with my single 6th gen AirPort Extreme, slow downs, sometimes to a crawl.
Normally, I would troubleshoot myself, but in this case, I figured that my AirPort Extreme was outdated, and it was time to move on to something new. Right before all the lock downs in the US in 2020, I bought a very expensive and highly rated Linksys Tri-band Mesh System.
I thought that with the Mesh system, I could expand my network range, and have internet in my parking space in my old home. They advertised that it would be seamless transition between nodes, but it ended up not working at all. Worst part, I still had similar slowdowns that I had on my old AirPort Extreme.
I spent several hours troubleshooting with Linksys tech support, which was helpful, but didn't provide any good solutions. This is what I learned about Linksys' mesh systems, as well as my own old AirPort system.
The slowdowns had to do with the amount of devices I had connected to the 2.4Ghz band, as well as congestion with my neighbors. I split the two bands on the AirPort Extreme, the performance on the 5Ghz band was actually better than the brand new mesh system's 5Ghz band.
Linksys hides their advanced settings. They have a lot more customization on the settings, but you have to basically do a factory reset, and set it up to take advanced settings from the start rather than setting it up like normal and then try to change the advanced settings. One of these advanced settings was splitting the two bands, which was Linksys tech support's advice to me to get the Linksys mesh system to perform better.
Seamless node switching doesn't work well at all. When I was sitting in my car in the parking lot, I was connected to the Linksys mesh system, but it stayed connected to the node that was the furthest to me. Linksys tech support's advice was to turn of my wifi on my iPhone and turn it back on to reconnect to a closer node. This was not what I was expecting.
I ended up returning the expensive mesh system. If I didn't have true seamless node switching, and I had to split my 2.4 and 5Ghz band anyways, I figured that I just would stay with the AirPort Extreme.
Here is the thing, after going back to the AirPort Extreme and splitting the two bands, the performance was amazing. So much so that I purchased another 6th gen AirPort Extreme on eBay and moved it closer to the side of my house that I parked my car, and I had great internet from my car.
I bought a single family house in 2021, and now have five different 6th gen AirPort Extremes all over my house and garage. I have excellent coverage through out my entire house, 3 floors, and outside my house as well.
I said all that for this reason, the AirPort Extreme 6th gen, while old, is still a great device that can hold up to even new technology.
maximum speed is around 180-200Mbps when in the same room. Even on Ethernet it's around the same speeds.
If you are getting that slow speeds on ethernet, then there is most likely a HW problem. I would swap your Ethernet cables to rule that out, but there shouldn't be any reason you are getting such a different from the ISP router over ethernet.
I have Gigabit service (not that I need anything that fast, but with cable it is my only choice if I want a decent upload speed), and get almost 1000Mbps on devices I test connected with Ethernet.
Are you using the same port on the ISP Modem for each router?
Have you tried a different ethernet cable?
Also, what device are you using to test the speed when doing a wired connection? Would this happen to be a Mac with a 10Gbps ethernet port?
