I get that it's fun to conjecture about things, but no serious scholars debate his authorship. You have to do some serious mental gymnastics to get past all the evidence that he wrote his own plays.
> he owned no books
No, we don't know that he didn't own any books. How would you even prove that? We just don't have a list of which books that owned. We also don't have lists of books owned by the vast majority of people who lived in Elizabethan England. I find this a particularly weird argument for someone who lived over 400 years ago.
> he was not contemporaneously referred to as a playwright or poet
There are literally 18 published plays during his lifetime, numerous editions of which list Shakespeare as author right on the title page: https://en.wikipedia.org/wiki/List_of_Shakespeare_plays_in_q...
Example: https://en.wikipedia.org/wiki/List_of_Shakespeare_plays_in_q...
In 1592, "Greenes, groats-worth of witte" refers to him famously as an "Upstart Crow" and makes fun of the fact that a mere actor thinks he can write. What better evidence is there than that?
In 1598, Frances Meres wrote a book about English poets called "Palladis Tamia, Wits Treasury" that talks about both Shakespeare's poetry and plays. It says "Shakespeare among ye English is the most excellent in both kinds for the stage" and then goes on to literally list his comedic and dramatic plays by name. I'm not sure how anyone can claim he was not referred to as a playwright or poet in his lifetime: https://internetshakespeare.uvic.ca/Library/SLT/life/early%2...
Numerous other contemporary mentions of him in documents are collected and available online: https://shakespearedocumented.folger.edu/resource/playwright...
What is definitely true is that plays during this time period were often collaborations between multiple writers. Different authors might add in lines or scenes. Two authors might co-write a play (The Two Noble Kinsmen) or numerous author might 'punch up' a work. Shakespeare primarily wrote his plays, but other people definitely contributed bits here and there, just like he contributed bits to other plays that don't have his name as author (like "The Booke of Thomas More").