Minä opiskelen suomea mieluummin aamulla, mutta toisinaan illalla.

mieluummin means “rather” or “prefer,” expressing a comparison between options.
mielellään means “gladly” or “with pleasure,” indicating you like doing something without comparing alternatives.

Time-of-day expressions in Finnish use the adessive case (-lla/-llä) to mean “in the _.”
• aamulla = “in the morning”
• illalla = “in the evening”
Using aamu alone would be ungrammatical in this context, and aamuna (essive) would mean something like “as a morning,” which doesn’t work here.

Yes, that word order is grammatically correct. Finnish word order is flexible:
• Placing mieluummin immediately after the verb highlights your preference as a modifier of the action.
• The original sequence (verb + object + manner + time) is most neutral and common.
All variants are understood, but slight shifts in emphasis may occur.

mutta means “but” and signals a contrast between your preferred study time (morning) and the occasional evening sessions.
• ja or sekä would mean “and,” simply linking two facts without that contrast.

• toisinaan = “occasionally,” implying somewhat regular but infrequent occurrences.
• joskus = “sometimes,” more general and colloquial.
You can use joskus illalla, but toisinaan sounds slightly more formal or literary.