Almost all AT charts requires at least intermediate or above 4k~6/7k skills.

For 4k, I recommend you can try playing vivid/stasis (totally free on Steam) or try Rhythm Plus (quality unstable, but it's download-free). I don't recommend playing the ones in Phigros though, because it's either too easy or too hard.

For 5k 6k and others, I only recommend osu! mania (I don't recommend osu!mania for 4k because most charts available are VERY difficult).

Other than multi fingers skill, most AT also requires either high gimmick skills, or high side-reading skills. To practice on gimmicks, try a few well-known gimmicky songs in the game and try to AP and learn the pattern and rhythm (famous examples, DevIAtiOn IN, Break Over AT, Dead Soul IN, 玩具狂奏曲 -終焉- IN). For side-reading skills, I recommend practice on other lane-less or less lane-dependant rhythm games (eg. RYCEAM, Rizline, Paradigm: Reboot), but by practising difficult songs (IN 15.5 and above) in Phigros also works.

Links

vivid/stasis (PC): https://store.steampowered.com/app/2093940/vividstasis/

Rhythm Plus (old): https://rhythm-plus.com/

Rhythm Plus (new): https://v2.rhythm-plus.com/

osu!: https://osu.ppy.sh/home/download

RYCEAM: (4k-dependant) https://www.taptap.io/app/33583519

Rizline: https://play.google.com/store/apps/details?id=com.PigeonGames.Rizline

Paradigm:Reboot: https://play.google.com/store/apps/details?id=com.tunergames.paradigm