Question about Ubiquiti EdgeRouter 4

Yes the device is suitable for this. L2TP Ipsec, pppoe. You can do almost everything with the EdgeRouter4. But you have to be aware that only the basic settings can be done via the web interface. Many things have to be set via CLI. I2TP as an example. You can find information at ui.com/support. Read the manual there and have a look at the configuration examples. There is also a large English community for the Edge products. If you don't know anything about CLI, it will be a challenge. Perhaps the unifi usg / usg pro / dream maschine pro would be the better choice, as everything can be set via the controller software. Take a look at the data sheet for both products and compare.

Yes, as the others said, CLI is very powerful and you can find many examples on the internet.

OpenVPN can also be configured.
But as OldZenki has already said, EdgeOS CLI knowledge is indispensable.