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MySQL and PostgreSQL are two completely different database architectures in the same way Windows and macOS are different. Our backup software only works with MySQL, for two good reasons: a. over 99% of Joomla! users use MySQL and b. only a minority of third party extensions supports PostgreSQL.

What you're trying to do is the site equivalent of buying a Mac and expecting to be able to restore a backup of your Windows applications on it with everything magically working. You can't, and even if you could it wouldn't.

I understand the context though. You are one of those who stayed with SiteGround, which won't support MySQL 8 despite the following facts:

1. MySQL 8 was officially released on April 19th, 2018 – SEVEN (7) YEARS ago. SiteGround has a very long time to prepare for it.
2. MySQL 5.7 is End Of Life since October 2023. It no longer receives security updates. Therefore, it's not fit for use on a live site regardless of the CMS or programming language used with it.

SiteGround has positioned itself firmly into the abysmal host category by only offering End of Life software on its servers. I strongly advise you to move your site to a different host. I currently recommend Rochen or CloudAccess.net. Full disclosure: Rochen hosts our business site free of charge but I also have a reseller account with them which I am paying for out of my own pocket, at full price. My recommendations are based on personal experiences; I will not recommend something I am not happy paying for with my own money, and I do not use affiliate links or any other kind of income sharing schemes.

As a side note, keep in mind that recommendations may change over time. There was a period of time in the 2010s when SiteGround was a very good host and we'd recommend them enthusiastically. That is to say, if you find a post from 10 years ago I may had been recommending SiteGround, but that was a different SiteGround, one with its own servers, a team of engineers carefully considering the configuration, support that actually knew the subject matter really well, and a good amount of community engagement i.e. the exact opposite of SiteGround in the 2020s which is a little more than an inadequate and obsolete dressing on top of generic, cheap, underperforming Google Cloud servers.