Play as low as you physically are able to or accommodate your set-up in order to go lower. Regardless of the game. It doesn't matter how good you naturally start at with one sens or another. Over the long term, I hold the strong belief that acclimating yourself to a lower one is ALWAYS better. What a low sensitivity does is increase your consistency. Even when you're performing poorly, there's a far bigger margin of error you're working with and will end up landing your shots anyway. There's no loss in speed or comfort at all assuming you're using a very big mouse pad and fast sweeping arm motions. Also, you need a light mouse. Over 95g is pushing it in terms of fatigue, honestly.
AimBooster/Osu is shit for training aim. You will literally only get better at playing AimBooster/Osu. You need something with an actual 3D engine and configurable FOV+sensitivity to match the games you're playing. Aim Hero on Steam is good for this. I can feel the difference in control over my tracking directly after doing a strafing target exercise on there, for example. Training drills with measurable results over time in the actual games you're playing is also good. Simply playing games normally without putting deliberate thought and action into improving your aim makes for really slow progress. I'd say, based on results and experience, that my aim in <1 year of playing competitive FPS is indistinguishable from professional gamers. It's on the same level of consistency or better at times based on the pro gameplay I keep up to date with. I've seen players with a decade or more of experience in the genre with much worse aim than me and I have to say it's precisely because of deliberate practice with a lower sensitivity than most.
If you're curious, I'm playing 57 cm/360 on a 90cm wide mouse pad at the moment. It's very comfortable and you wouldn't even be able to tell spectating me that I'm playing so low in Quake Champions+Overwatch with my violent 180's and such. There's enough real estate on the pad for me to go even lower, as well.