The obvious answer that you might not like very much is "write a paper and co-author it with an established group." If you share a lede with an established group, that's immediate credibility.
Journals happily accept manuscripts from first/corresponding authors who don't have PhDs. Graduate students publish regularly; depending on your program this is effectively obligatory.
Self-funding publication in a peer-reviewed journal will raise eyebrows -- so see my first suggestion, which offloads the often substantial publication costs onto a grant. (Our group's last publication cost was ~$4500, for a benchmark. That's higher than normal but it was open access with 6 color figures). Crediting a grant for the work also lends credibility; someone gave you cash to do this thing, so obviously they don't think you're totally nuts.
Publishing preprint-only on the arxiv is well established. I think you need someone to vouch for you the first time, but that shouldn't be too hard depending on the field.
Depending on the field, single-author maunscripts are either basically normal (parts of CS, much of mathematics, some theoretical physics), a giant red flag (experimental physics, biology-adjacent fields), or somewhere in between (economics?).
Do not ever use a vanity publisher for scientific articles. If the journal ever appeared in Beall's list, run, because it's a mark of shame.
Finally, don't necessarily attribute your marginalization to not having a PhD -- the politics of publication are ruthless no matter who you are.