I’ll start by saying that obviously the following suggestion wouldn’t be necessary if the limits weren’t in place, and will obviously require more manual effort for you, but if you’re ok to work around, this might help you at least in the short term. I’m also not sure if this will help your specific case but some things to try.
If your issue lies in losing the side effects that come with the watchlist (such as being included in calendar), you could create a list for items (whether movies and/or shows) that you know don’t need to be in the calendar or up next right now.
eg.
Sort your watchlist by release date, and move all Released movies to a separate list for later, and leave all unreleased movies in your watchlist. Obviously you lose the ‘auto-remove on watch’ from the extra lists, but otherwise you can always sort the other lists periodically by watched status or watched date and delete those that are watched, etc. This is something I do myself with a list for my partner and I to keep track of things we want to watch together and, using the newly-made-free feature of adding list collaborators, even free users can both add and manage the list items when necessary.
Same as above could be applied to your shows that are otherwise already released or canceled/ended that you know won’t be appearing in your calendar in the future.
For everything else, it’d likely be about prioritising those which you care to actually be alerted about in calendar that you haven’t started watching yet and keep those in your watchlist (are you really going to get to all of those?).
Your other options would be to utilise the collection feature, up to its limit, as things will show up in calendar for those. Or worst case you could also, as a ‘hack’, have a list of shows that you add a single episode to your history way in the past that you intend to watch at some point - this will cause the show to come up in the calendar without needing to be in the watchlist.
Again, sucks that any of that might be necessary, but if VIP is not an option for you then that’s about all I can think of to help workaround the issue.