Games like Mandala
Games like Mandala include Nova Drift, Chippy, GRIS, Touhou Koumakyou: New Classic - the Embodiment of Scarlet Devil, and Bit Blaster XL. We matched them by the tags players give Mandala on Steam (Arena Shooter, Bullet Hell, Shoot 'Em Up, Colorful, and Psychedelic): closest matches first, and among equally close ones the better-known games.
Nova Drift
Shared: Bullet Hell · Shoot 'Em Up · Arena Shooter
Chippy
Shared: Bullet Hell · Arena Shooter · Shoot 'Em Up
GRIS
Shared: Atmospheric · Relaxing · Female Protagonist
Touhou Koumakyou: New Classic - the Embodiment of Scarlet Devil
Shared: Bullet Hell · Shoot 'Em Up · Female Protagonist
Bit Blaster XL
Shared: Bullet Hell · Shoot 'Em Up · Shooter
SNKRX
Shared: Arena Shooter · Shoot 'Em Up · Top-Down Shooter
Picayune Dreams
Shared: Bullet Hell · Colorful · Psychedelic
Planes, Bullets and Vodka
Shared: Shoot 'Em Up · Bullet Hell · Shooter
Time Wasters
Shared: Bullet Hell · Arena Shooter · Shoot 'Em Up
Sektori
Shared: Shoot 'Em Up · Arena Shooter · Colorful
20 Minutes Till Dawn
Shared: Bullet Hell · Shoot 'Em Up · Top-Down Shooter
Touhou Juuouen 〜 Unfinished Dream of All Living Ghost.
Shared: Bullet Hell · Shoot 'Em Up · Shooter
Titan Attacks!
Shared: Arena Shooter · Bullet Hell · Shoot 'Em Up
Beat Hazard
Shared: Bullet Hell · Shoot 'Em Up · 2D
TouHou Makuka Sai ~ Fantastic Danmaku Festival Part III
Shared: Bullet Hell · Shoot 'Em Up · Female Protagonist
State of Anarchy: Master of Mayhem
Shared: Bullet Hell · Shoot 'Em Up · Arena Shooter
ZeroRanger
Shared: Shoot 'Em Up · Bullet Hell · Indie
BALL x PIT
Shared: Bullet Hell · Shoot 'Em Up · Top-Down Shooter
Touhou Kinjoukyou ～ Fossilized Wonders.
Shared: Bullet Hell · Shoot 'Em Up · Shooter
Sky Force Reloaded
Shared: Shoot 'Em Up · Bullet Hell · Top-Down Shooter
Waves
Shared: Bullet Hell · Shoot 'Em Up · Shooter
Touhou Tenkuushou ~ Hidden Star in Four Seasons.
Shared: Bullet Hell · Shoot 'Em Up · Female Protagonist
Everhood
Shared: Bullet Hell · Psychedelic · 2D
Vampire Survivors
Shared: Bullet Hell · 2D · Indie