The outlines don’t do well when you have straight edges, and when you do add an outline the shadow doesn’t make a shadow of the outline. If that doesn’t make sense then hopefully the images does (alt 1 and 2 were taken from different programs)
I feel your pain but the Titler app within Kdenlive needs a complete rewrite. Some work was done about two years ago as part of the Google Summer of Code (sic?) but left unfinished.
AFAIK, it is on the 2-to list of the developers. But bug fixing and stability and performance improvements are the focus for the upcoming minor releases.
BTW something i’ve been doing is using a different font “Alte Haas Grotesk” which looks like a rounded Arial (just a bit different)