@mp10 

Hey!  Hopefully be able to answer some of these for you

1. Can the Archer VR900 hardware V3 and Archer VR2800 hardware V1 VDSL Modem Routers be configured to operate as a VDSL modem only, by totally turning off the router function?
If so, what need to be changed in the router configuration settings?

No it cannot, the router function is required to be active.  What was your desire / idea with modem only mode?    It should not be needed

2. Which hardware combination is likely to provide the best overall Wi-Fi performance (highest data speed, lowest latency, lowest data packet error losses, having to reboot, etc), perhaps list in order:

    a. The Archer VR900 V3 modem router configured to be used as a VDSL modem only (presuming it's possible), and connecting VR900 to a dedicated 'Wi-Fi 6' or 'Wi-Fi 6E' capable router.

    b. The Archer VR2800 V1 modem router configured to be used as a VDSL modem only (presuming it's possible), and connecting VR2800 to a dedicated 'Wi-Fi 6' or 'Wi-Fi 6E' capable router.

    c. Using the Archer VR900 V3 or Archer VR2800 V1 as a combination VDSL modem Wi-Fi router, as it was designed to be used as. I guess this might eliminate any latency, compatibly issues, etc of connecting the two devices together, and using the modem router as it was designed to be used (as a combination modem router).

    d. Connecting together a dedicated VDSL modem (no built-in router) and a dedicated 'Wi-Fi 6' or 'Wi-Fi 6E' capable router.

    e. Or there's something else that's better than those options!?

Sounds like basically you want the quickest PING you can get..  it seems you could be overcomplicating the solution. 

VDSL modems are gone now, really not of any use anymore, and have been replaced with VSDL Modem Routers, similar to what you have mentioned.    But this may not be your best option.

Speaking honestly, with the download / upload speeds you have from Talk Talk.. unless you live in an area flooded with WiFi then there is little advantage going WiFi6.   WiFi5 speeds can get solid 450mbps which is much faster than you need.  WiFi 6 also only works if devices connected to it are WiFi6, if you mix 5 and 6 its not great to be honest.
 

If you want just best wireless available, don't go the home range..   Keep your current Talk Talk router and add in an access point, turn off the router WiFi.   An access point will massively improve your wireless signal, especially something "business grade"

Check out either the EAP245 (wifi5) or the EAP620/660 (wifi6).   Cable this into your talk talk router, turn off the talk talk wifi and use this instead for wireless...  As this is business grade, its better quality and performance overall.  These are designed for 60+ wifi devices running constant so offer the best quality of service that you want.

I have 2x EAP245s in my house, each has 20+ devices connected to it and they can handle 450mbps easy on my 1GB BT FTTP.   One of them would be overkill for you on a 75 down connection, an EAP6xx would be even faster but more expensive.

@Philbert 

Thanks for your advice :-)

The modem only mode idea was to be able to add Wi-Fi 6 or Wi-Fi 6E features from one of the latest routers due to the VR900 or VR2800 not have them.

I don't live in an area flooded with WiFi so I guess no major advantages switching to Wi-Fi 6 then.

I currently have a VR900 V3 connected to the TalkTalk VDSL2 Internet service. I removed the TalkTalk router (Huawei DG8041W) a year ago. An occasional issue I have with the VR900 is that it needs rebooting every few months due to the Internet connection dropping - Rebooting reconnects the Internet every time so I guess the problem is very likely the VR900?

Using the VR900 router, ping is 6ms on WiFi, 5ms on Ethernet. Download speed is 8Mbps slower on 5GHz WiFi.

I tested the TalkTalk router speeds and all were similar to the VR900.

Do you happen to know if swapping the VR900 to a VR2800 V1 (I have, but not installed) would have any advantages? Possibly the Internet connection dropping.

@mp10 

To be honest, they are quite similar routers and the vr2800 is just the 900 on steroids.. its got a faster processor, more capability and better WiFi so yeah would be a good replacement / upgrade.

To have to reboot it every few months seems perfectly normal, I have my router on a reboot schedule every 2nd sunday at 4am...  been doing that for years without issue.  If you are getting a few months then that's well above normal.    However the extra CPU and resources of hte 2800 may help that also, however, I would recommend you consider a reboot schedule!

The VR2800 should do everything you require mate, likely a worthwhile change!