I actually think it's best to play them in the order of release. You'll appreciate Berseria more with knowledge of Zesteria. And yeah I also thought Zesteria to be pretty poor in comparison to the other Tales games. Just didn't particularly enjoy the characters and the way you are forced to use only one way of attacking a given enemy. Berseria is better and, in my opinion, Z is best as a groundwork / reference point that is played before B.
Zestirias plot certainly only is explained in the secret dungeon of berseria which is pretty pathetic. And since its never resolved you are left with an unfinished little group of games. On top of both of them being pretty disappointing with how they end to begin with. However berseria has minimal connects to zestiria and the couple it does have are nothing more than amusing foot notes at best. They also create some plot holes like Rocko not being in zestiria or explaining what happened to him between games. Magilou they just write off as having died between them with no explanation for how that happened either and you wouldn't know who she even is in zestiria so that whole section about her felt like nonsense. Oh and the couple cities and temples existing in both games despite the entire planet changing terrain between the two games. The wind temple shouldn't have still existed in zestiria as theres no way it would have survived volcanoes and the land all moving and shifting all around it.
Given that magilou is immortal barring she breaks her pacts I feel like they wrote themselves into a corner backwards with berseria and didn't put much thought into how they would proceed.
Oh And Zaveid is such a different character between the two games and the only named one that you would actually even meet between the two that they might as well have not wrote him in at all. Going from someone who would never kill to someone who kills without even a second thought.
Then there is the fact that edna, lailah, thunder spirit and a few others are not present in berseria period and you never visit not japan where rocko is from.