Is there a way to put an "entrance page"  in front of the forum rather than re-install it into a sub-folder below the top URL address?

Thanks!

You can do that with the Pages app. 

Or just blocks, where you hide content from guests and show your blocks to guests only. There is even a “Guest Signup Widget” included with the software. It may not exactly do what you want, but it is made for the purpose you ask about. 

Another option would be this one, graphically a bit more sophisticated than the welcome widget and with many settings...

https://v0rt3x.dev/files/file/62-hero-image/

Blocks will show only if the content they contain can be seen by the viewer. So in the case of these, they are members blocks, gallery blocks etc. So you would need to remove permission from guests to view those items in order to remove the blocks. Looking at module permissions may serve you better for some

https://invisioncommunity.com/4guides/how-to-use-ips-community-suite/managing-members/member-permissions-r305/