Google Analytics Import: choosing the import method, custom events, custom dimensions, views

Hello, I read in the help that for high-traffic websites it is not advised the Quick Connect and Google Oauth configuration is to be used.
What do they mean with high-traffic websites? I have 300k monthly pageviews. Is this high or low? Also, does it matter how many years back were data collected? I have tracked the websites since 2019. Should I use quick connect or not?
Furhermore, I have custom events. Will they be imported?
And what about custom dimensions? I have 10 in GA3, can I map all of them in Matomo custom dimensions?
As for different views, will be they imported each as a different website?