… how to make the lines look natural, moving from one system to the next …

What is the problem? It is not clear from your description.

It might help if you attach a graphic showing the problem.

The better we understand, the better we can help.

There are two ways to do this in Finale. The first is to fake it—you're really writing modern notation but it looks old-ish. This page in the User Guide will get you started.

https://usermanuals.finalemusic.com/FinaleMac/Content/Search.htm?q=plainchant 

The other is to use a plainchant font to create actual plainsong. Medieval 2 is the one that works with Finale 25/26/27.

Robert Piéchaud Releases Medieval 2

https://www.finalemusic.com/blog/robert-piechaud-releases-medieval-2/ 

Apparently, Medieval 2 is no longer available. I've no idea what to use in its place. Cantorum has been released for Dorico and looks similar but I don't know how/if it works in Finale.

https://www.notationcentral.com/product/cantorum/ 

Here's another set of links from the manual for faking plainsong.

https://usermanuals.finalemusic.com/FinaleWin/Content/Finale/Chant.htm?Highlight=chant