Hi, thanks. I have already have L2TP/IPSec server running and working in my openwrt router. In-fact, I'm not using the two ways(WireGuard or OpenVPN) you mentioned. Instead, I use Softether VPN server. The setup is also not complex(see How to tweak the firewall of OpenWrt 22.03 to support Softether VPN server - Installing and Using OpenWrt / Network and Wireless Configuration - OpenWrt Forum and L2TP vpn client report 720 Error when try to connect SoftEther VPN on OpenWrt), in-fact, it is quite easy. I have seen the OpenWrt wiki doesn't have a page for the Softether VPN server, but I don't have the right to edit the wiki. I have try to applied a wiki account several weeks ago, but failed(reply got deleted automatically). Today have tried again. (see here: Applying for OpenWrt wiki account)
OK, thanks. I will consider your advice.