-150ms is a huge difference compared to 0ms local offset. The hit error results you got before changing the offset doesn't suggest a change of offset so huge; it seems like you are timing hits visually instead of auditively, so you should change the hit position instead of the offset.
For adjusting the offset, it's better to play a well-timed easy map you can do with your eyes closed, and then change the value according to your results (for local offset, decrease the value if you are hitting early, increase if you are hitting late; for universal offset, increase the value if you are hitting early, decrease if you are hitting late).
For adjusting the hit position of your osu!mania skin, play a map with sound off (ideally with a skin with invisible hit judgments, and no hit error bar), and adjust the hitposition based on the results (increase hit position if you are hitting late, decrease hit position if you are hitting early)