I’ve been using renoise for a while but still have oceans to learn about it.
just am wondering if there are any tools specifically for portamento / legato that have more control + a decent UI ?
Yep. Paketti. Just look for Paketti Glider after you download it 
Legato would be placing one note below another in the same subcolumn. Depending on how the native instrument or VST is configured, that would already sound like legato.
Portamento would involve doing the same as legato but adding the parameter 0Gxx to the second note to “merge” it with the first, modifying the pitch between them. It would simply be playing with the range. What would a tool have to do here? Add the parameter and use a slider to change the amount or something like that? Something like that already exists in the Phrase Editor.