There is actually a good grammar and spell checker app for the mac that has been around for a very long time. (since the 90s I think.) I very rarely hear it mentioned. I works as an app that can copy and paste and spell check an entire page.
It also installs as a grammar and spell check service. It can replace the built in MacOS grammar and spell check service and be used anywhere that the built in service can be used including scrivener.
It is a one off purchase no monthly charge. It is called Grammarian Pro 3.