Just to add, people in the field are unlikely to find a paper on Zenodo. I don't think any of the major search engines or databases for papers will include anything on Zenodo in their results.

That said, posting on arXiv can't be done unless someone vouches for you, which might be difficult or not for an independent person.

I think the best bet would be to submit your paper directly to a journal. However the paper in GP is unlikely to be published by any reputable journal. One direct feedback: if you can't explain at the start why your paper is relevant to current researcher's then why should anyone care? A sniff test for this is that you discuss in the introduction recent papers which have tried to solve the same or similar problems. But GP paper's references are over two decades old.

Getting someone to endorse you on arXiv is easy if you have work to show. (If you don't get endorsed, it's theoretically possible that the endorser might plagiarise your work, but you'll have a paper trail to prove it; I've heard more stories of PhD supervisors plagiarising their students' work than of arXiv endorsers doing so.) It's hard for us plebs who don't yet have work to put on the arXiv to get endorsements, but that's not actually much of a problem.