You know how to hide barlines in Encore 5, I trust. Overture 5 lets you Delete Barlines and supports Bravura – a new standard SmuFL font from Steinberg. Overture does not have a figured bass tool and for that reason, I’m not expecting to see it in Encore 6. I have no idea if Bravura or SMuFL fonts will be supported but many have renaissance glyphs..
If you are looking to fake chant/plainsong (modern notation that has the look and features you describe), Finale can handle that easily and there are many old looking fonts you can use, some are free plus the renaissance glyphs in many SMuFL fonts. Dorico has some of this functionality, supports SMuFL and both have Figured Bass tools. Finale has lowered their pricing recently and you might get competitive upgrade pricing as an Encore owner—I would write and ask.
If you want actual plainsing (stackable neumes etc.), many use a vector drawing app such as Illustrator. Otherwise, the only real notation app to consider is Finale with Robert Pichaud’s Medieval 2 plugin.
https://www.klemm-music.de/notation/medieval/en/index.php