I found that the switch key of the second generation elite handle configuration cannot be...

They don't do it probably because the switch key already have a unique function for them. A a sufficiently important and non-modifiable function for them that the possibility of doing so did not appear to them as viable

If you want this feature, then make a post about it on the ideas site here
https://www.reddit.com/r/xboxinsiders/wiki/ideas

This website cannot open, probably because I come from China

I mean, the configuration switch key can be defined on other keys, so it's easy to switch, and the configuration switch key doesn't change its function

Keep 2 precision weapons when facing elites and switch to the other weapon when his shields are down for a quick kill. For promethean knights use all available weapons to take it down, I use various combos such as sniper + precision weapon, automatic weapon
+ precision weapon, shotgun + grenades. Trial and error is the key to handle tough situations.