Installing SSL is much simpler now with Let's Encrypt, but there's still a number of other hurdles & pitfalls for site owners to move existing sites, such as fixing mixed content and similar issues.
That's true, thanks for noticing this mistake!
I've now corrected the post to say only DV & OV certificates are available as wildcard and EV allows multi domain but not full Wildcard.
This is very true, auditing before move makes fixing all these mixed content issues a lot easier. There's some tools in the above guide that can help but there's not really a perfect solution yet.
Completely agree, with movingtohttps.com I'm hoping to ease the process. It's open source if anyone would like to contribute to improve the guide too. But there's still many steps that are difficult that perhaps better tooling could help with.
HTTPSChecker have a desktop app (windows, mac & linux versions available) & an online service.
They can be used to scan sites for mixed content and similar issues, along with monitoring for issue reports after migration.