I love how the music slowly changes as you get more upgrades! The solo mechanic is really cool in theory, but WASD and 12345 don't work together that well - maybe if the attacks had autoaim, leaving the mouse hand free (not having to use the mouse), you could use keys that the player can easily press with the right hand.
If I were to make this game here's how I would try to make it:
- Have the "solo" challenge going all the time, but instead of a specific sequence, each instrument/upgrade you pick up has a different note it adds to the beat, which you have to match with the corresponding button - for example Bass would play one note every 1.5 seconds, where you have to press [2] to trigger its attack. Then each new instrument adds an additional "note" to this sequence, so the more upgrades you have, the more complex the melody becomes. Maybe multiple upgrades of the same instrument type could change the rhythm, for example two bass notes in quick succession instead of one at a time.
- These attacks should automatically aim at the nearest enemy
- Move the rhythm bar to the middle of the screen, like in Crypt of the Necrodancer