No WPA3 for apx, no fast roaming between apx and ap6

sry, my router is only 8 years old.

It depends on the hardware chip, you are using. APX is 6 years old. Your Router might have different chips than the APX. 

I am not sure, if a comparison is applicable. 

Additionally, WPA3 Fast Roaming is a topic for a lot of the vendors. You find a lot of articles about how to approach WPA3 / WIFI6 with older Access Points. 

it already starts with duplicating all configurations instead of just applying possible settings. wpa3 is a software implementation and not a hardware implementation. there is no reason to not have it for apx. especially the more powefull models.

same with roaming. there is no reason why the accesspoints dont talk the same protocol accept intentionally in software.
google 802.11k, 802.11v und 802.11r

Right now, we are looking into bringing an Option to copy the settings from APX to AP6. 
WPA3 is a requirement for WIFI6 - You cannot release a WIFI6 access point without WPA3 support - Thats what Sophos did. 
The development of bringing WPA3 to APX is still considered but not planned yet: Little Question: Why no WPA3 Support for APX?
About roaming: You will introduce some problems with Roaming between different WIFI Standards. It boils down to the Client and how the client interacts with the Wireless SSIDs. 
We wrote an Article about htis: https://support.sophos.com/support/s/article/KBA-000008455?language=en_US

You can find also other vendors talking about the same challenges. 

i dont want to roam between wifi standards. wifi2 roaming would be ok, support says still not possible.
i just asumed i get wpa3 with less than a year old devices. stupid me

Did you try to create a WPA2 SSID for AP6, then create the same name of WPA2 SSID with APX and try to roam between those two? 

Again: If you have an SSID with WPA3 and another SSID with the same name and WPA2, it will break the roaming in most cases, as devices will not follow this roaming behavior. 

i better not risk it. the guide for mixed environments says no. support says no.

The encryption incompatibilities usually come through the WPA3 vs WPA2. You could try it. 
If you have an AP6420E, you need to be careful about the 6GHz - As 6GHz is only supporting WPA3 (per standard). 

if i have to change all ap, have to configure everything again. what benefit do i have in staying with sophos hardware?

I cant follow here: You can manage all Sophos APs with a centralized approach (Sophos Central or SFOS for APX and Central for AP6). 
What do you mean by you have to change all APs? 
My approach would be: 
Lets assume your current APX SSID is called "Sophos" for 2.4 and 5Ghz - WPA2 Personal. 
Create another 2.4 and GHz SSID for AP6 and call it "Sophos". Choose only to use WPA2 Personal. 

Check how the roaming works from APX to AP6 and vise versa. 
Next steps would be: See where you place AP6 and if there is even a roaming scenario. If not, you could move to WPA3 for AP6 as well. 

The benefits of Sophos Hardware would be to have it centralized managed. 

I cant follow here: You can manage all Sophos APs with a centralized approach (Sophos Central or SFOS for APX and Central for AP6). 
What do you mean by you have to change all APs? 
My approach would be: 
Lets assume your current APX SSID is called "Sophos" for 2.4 and 5Ghz - WPA2 Personal. 
Create another 2.4 and GHz SSID for AP6 and call it "Sophos". Choose only to use WPA2 Personal. 

Check how the roaming works from APX to AP6 and vise versa. 
Next steps would be: See where you place AP6 and if there is even a roaming scenario. If not, you could move to WPA3 for AP6 as well. 

The benefits of Sophos Hardware would be to have it centralized managed. 

You did not know?
You have to make separate SSID configurations for APX and AP6. That means i have to configure everything again and have everything doubled in the interface. Fun to manage.

I wouldnt call sophos central a benefit. its quite limited if compared to a local controller

i know that i am somewhat just ranting, but being forced to exchange quite a number of accesspoints has direct influence on my budget. just because sophos decided to not make their devices fully compatible with each other, when i have much much cheaper devices that just updated.