I would go ADU or if you live in a desirable area then list it on AirBNB.

BUT, your biggest issue is going to be converting it into a livable unit and adding a sewer line if one is not already there. You could convert it to a studio which would save you some renovation cost but will also lower your rental rates.
I would get quotes to finish the apartment. Then you could split the garage with the unit so you keep 1 space for yourself to either use for storage or parking or whatever and the other space goes with the rental. The ADU will also increase your overall property value.

Do you have a full time job outside of the consulting business? 10-15 hours a week to result in $175k+ a year in income is pretty darn good.

Since you don’t want to expand the business, I think your next biggest selling point would be your client/contact list. Another company doing something similar that you do might want that information, but I don’t think they would pay 2.25x simply for the list. They could, but it would be more of an acquisition rather than a buy, at least I would see it going that route.

I think a 1 man consulting business would be a hard sell. You ARE the business and your consulting expertise is the reason your business creates the revenue it does. Personally, to make your business more attractive to buyers, it would benefit you to hire 1-3 employees. Teach them everything you know and increase revenue, this will prove to potential buyers that with the right training the business can grow without having your knowledge/expertise in place. Also, if your newly trained employees increase revenue X times and you get to decrease your time spent in the business, you might want to keep it longer and collect the profits.

Just a thought from an outside perspective.