At the GM's discretion.

The size is appropriate. The willingness is also clear since the spell specifically states that the caster or another designated creature can ride the phantom steed.

The condition of appropriate anatomy is rather vague which may be because it is rather clear what animals / monsters a human(oid) can ride on. It is unclear, however, what the appropriate anatomy is when the mounted combatant is a mule. "Horse-like" and using the Riding Horse stat block indicate that the appearance cannot be completely freely decided.

In the end, however, it is up to the GM to decide how far you can deviate from the standard horse and, more importantly, if "appropriate anatomy to be ridden by a mule" is a sufficient description of the appearance. The GM is clearly within their right to have you describe how the anatomy is appropriate.

At the GM's discretion.

The size is appropriate. The willingness is also clear since the spell specifically states that the caster or another designated creature can ride the phantom steed.

The condition of appropriate anatomy is rather vague which may be because it is rather clear what animals / monsters a human(oid) can ride on. It is unclear, however, what the appropriate anatomy is when the mounted combatant is a mule. "Horse-like" and using the Riding Horse stat block indicate that the appearance cannot be completely freely decided.

In the end, however, it is up to the GM to decide how far you can deviate from the standard horse and, more importantly, if "appropriate anatomy to be ridden by a mule" is a sufficient description of the appearance. The GM is clearly within their right to have you describe how the anatomy is appropriate.