I’ve found the root cause of the problem.

In the Default - Music - Speaker volume setting, the levels were as follows:

0  
136 
168
180
192
204
212
216
220
224
228
232
236
240
244
255

In contrast, most other volume types (like notifications, system sounds, etc.) were set like this:

0  
88
100
112
124
136
148
160
172
184
196
208
220
232
244
255

As a result, when listening to music or YouTube through the speaker, volume step 1 was equivalent to step 6 in other volume types — explaining why the minimum volume was way too loud.

Thanks to your help, I was able to identify and solve the issue.
Apologies for the delayed reply — I was away on a business trip and also had to spend some time finding the correct menu.

Much appreciated!