I'm not exactly an expert, but I am pretty sure the conjugation of the verb "be" in the second personal informal tense would be "art" as in "Thou art a sovereign citizen".
I could be wrong, but that's what they say in persona 5 like a billion times: "I am thou, thou art I", and in all other contexts, I have never once seen the conjugation "Thou is" (other than on this website). That just seems grammatically incorrect and forced to make a point about plural vs. singular tenses. So, what I am wondering is, is there a modern reference on how to conjugate verbs with `thou`? I'm pretty sure some English dialects still use it (just not in the United States AFAIK).
Since we say "He is", I use "Thou is". The verb tense "is" seems to align with singular tense. Whereas "are" is used for all the plural forms. I don't know about "art". Maybe I need to read more shakespeare.
A buymeacoffee donor referred me to this book by Quaker who defended the use of thou.
A battle-door for teachers & professors to learn singular & plural you to many, and thou to one, singular one, thou, plural many, you: wherein is ... between singular and plural (1660)
