Another way to do WiFi calls is to use a VoIP provider who provides SIP and call forwarding at the same time, or if the SIP fails. That phone number will not be able to receive texts, but you will get a call over WiFi if it is available, and failing that, it will forward to your cell number.
WiFi 6 will make WiFi more suitable for voice in crowded bands, and WiFi 7 will improve upon that even further. The Librem 5 uses Wi-Fi 4 (802.11n). I hope that the next phone skips to WiFi 6 as it will bring battery life improvements, and the improvements in WiFi 5 are more suitable to power and bandwidth hungry desktops/laptops.