Forum

As you already said, sadly the DMGuild is very restrictive on new settings being pubblished so that's not an option.

Honestly beside your official site or Drivethrurpg I don't think there are many other platforms in which you can reliably share the PDF. Ofc Kickstarter can be an option as a sort of preview or quickstart rules document, but I suspect that you will wait after the trial baloon is released and how is it well received to decide about going full on the project.

But I know some fairly frequented subreddits (for example r/dndnext) and forums (Giant in the Playground) about D&D, in which you can share the link for Drivethrurpg or your site (especially if it's a Pay as you Want or free document) to better spread the news about Symbaroum 5e ruleset and better evaluate the community reactions. Also the EnWorld site write a lot of articles about news on rpgs and I think they would be interested about a possible 5e ruleset for Symbaroum if you let them know about it. I will surely post the link to the "trial baloon" on several of the above sites once you release it, because I wanted to spread the knowledge and love for the Symbaroum game, which I really, really like.

Also, I know a lot of people which are fascinated by Symbaroum as a setting (people are sick tired of the Forgotten Realms ahahahah) but are not interested in the game system (mostly for lazyness tbh), so based on my personal experience I think that the trial baloon, with a little "marketing" done right, can be really good for the game as a whole.

Several good tips right there, thank you!
We will share it free or PWYW, as we want to pike interest and get feeedback from as many as possible.

Happy to help

One place to spread the word of Ruins of Symbaroum might be D&D Beyond's forums.

D&D Beyond has also cooperated with companies outside of Wizards of the Coast, perhaps they would be interested in doing something together with Fria Ligan. I know they made an adventure together with Riot Games for their Runeterra setting.