This post shows how to migrate from Google Analytics to (self hosted) Countly, and what the pros/cons/consequences are.
https://thomashunter.name/posts/2018-12-28-migrating-from-google-analytics
Having just migrated from Google Analytics to Matomo (formally Piwik) it's great to see that I'm not alone.
While I have removed GA from a swathe of (smaller) sites, landing pages and apps I have left it on our main site after to a discussion with a colleague.
While I have never hired an SEO, my colleague said that they all love to see GA data. It's not that's it's better so much as all their tools are written for GA. I also have over 8 years of historic data in GA which seems silly to throw in the bin and I feel is worthy of running the two side by side just for my main site.
I am using Simple Analytics on my product UpStamps. Very simple to use with a great mission on privacy.
@gorkemcetin The main problem for me is that if you use different services you get different numbers. For me, the difference was between 10% - 20% which is quite a lot (Matomo/Clicky/GA).
The other question is the performance. I mean if the small VPS that hosts Countly can handle traffic for an e.g. site with 200K uniques/day.
Expect 20%, as many ad blockers do block Piwik Pro / Matomo / GA already.
In your case you have 6M sessions, that can be handled by a medium-sized VPS ($5 DO instance won't cut it)