I believe one of the issues with apps that only front-end iCloud calendars is notifications. I use Weekview (bought years ago and ignore the nags to subscribe) and the free version of Calendars (will purchase if I continue to like it) and neither can handle event notifications. They'll always come from the stock calendar app and clicking on one opens Apple Calendar.
I don't mind this, as even if one hosts their calendars with e.g. Fantastical, the notifications won't work if you don't have a signal or are in airplane mode.
I also have that with the WeekCal app that my wife and I are using for years (every since Apple changed the layout of Apple Calendar) and with that one we also don't have a subscription (just bought once) and the notifications are only coming from Apple's calendar since this also synchronizes with our Google calendars that we use within this WeekCal app.