Since moving from Canon, Nikon and Leica after 40 years using them all, to Sony in 2017, I have owned Sony A6500, A7RII, A7RIII, A7III, A9, A7RIV, A7SIII, and currently A1, A7RV, A9III (yes today).
The Sony A9 was the first fullframe stacked sensor mirrorless camera with a then top 20 fps ability. It is still an excellent camera today.
IMHO I would get a good used A9II which had some improvements over the A9.
https://mirrorlesscomparison.com/preview/sony-a9-vs-a9-ii/https://cameradecision.com/compare/Sony-Alpha-A9-II-vs-Sony-Alpha-A9A9II are at bargain prices for good used ones right now, so that would be a great camera to start your Sony journey with.
And yes Sony's E-mount is the most widely used ,and the most widely produced mirrorless-designed lens mount in the world, no other mirrorless lens mount comes even close, none and that includes Canon RF.
E-mount has over 200 quality E-mount choices and growing every day from Sony, Sigma, Tamron, Zeiss, Samyang/Rokinon, Tokina, Viltrox, Laowa, Meike and more.
Sony and Sigma both are about to announce some more great new E-mount lenses on Feb. 21st.
https://en.wikipedia.org/wiki/List_of_Sony_E-mount_lenseshttps://en.wikipedia.org/wiki/List_of_third-party_Sony_E-mount_lensesAnd five of the six top Worldwide News Services have moved exclusively to Sony mirrorless for all their staff photographers and staff videographers worldwide, including Associated Press; Canadian Press; all Gannett media including USA Today; UK's top News Service.
AFP (Agence France Presse) helped develop and test the Nikon Z9, so they use Z9/Z8 for their staffers.
None of these top News Services chose Canon mirrorless for their staffers, none, just a fact.
https://alphauniverse.com/stories/why-the-associated-press-just-switched-to-sony/https://petapixel.com/2021/11/17/sony-is-now-the-exclusive-camera-provider-for-gannett-and-usa-today/https://www.dpreview.com/news/4545693607/the-uk-largest-news-agency-partners-with-sonyhttps://petapixel.com/2022/01/31/canadas-largest-news-organization-moves-exclusively-to-sony-cameras/#:~:text=Canada's%20Largest%20News%20Agency%20Moves%20Exclusively%20to%20Sony%20Cameras,-Jan%2031%2C%202022&text=The%20Canadian%20Press%2C%20the%20largest,provider%20for%20the%20media%20company.
https://petapixel.com/2022/06/09/how-pro-photographers-helped-make-the-z9-from-prototype-to-flagship/Sony leads in overall mirrorless sales and fullframe mirrorless sales for many good reasons. Latest BCN and CIPA numbers show that.
Cheers and best to you.