Specifically for EX users, 1.5.4 introduced better Onion Skin customization. You could finally tint previous/next frames with custom RGB colors rather than just the default red/blue. The Timeline palette also received a "Hold" function for keyframes, allowing animators to pause a drawing on screen for several frames without duplicating cels.
One common complaint about CSP 2.0+ is the heavy background telemetry and asset loading. In 1.5.4, 3D models load locally and instantly. Clip Studio Paint EX 1.5.4
Some resellers on forums sell perpetual keys for version 1.x. Ensure the key has never been used on more than two devices (CSP’s old limit). Specifically for EX users, 1