I did some investigation after reading the OP's question due to similar thoughts.
What I found is it would be best to put the question directly to the BATF.
Your "cleaning business", whether part-time or full-time borders on "gun smithing" and would require an FFL.
Some say as long as the gun owner is on the premises (you clean while he waits) a license would not be required, if the firearm is left over night you do need a license.
To many "what if" scenarios, only the BATF can answer.
A FFL is not that difficult to aquire from what I have read.
But here is something I learned when a LGS (local gun shop) owner decided to close his shop and retire.
He got out of selling guns but still wanted to do transfers and sell reloading components.
He closed the shop and set up his garage as his "place of business".
I was needing a transfer done, so I contacted him and was told he let his FFL go but was still selling reloading components.
OK, forget the tranfer, what powders do you have on hand and when can I pick it up?
Go to pick up the powder and he is out of town, so his wife takes me out to the garage to get the powder.
Discussion turns to why he let his FFL go.
Did you know the BATF can come to where your business address is, totally unannounced, and search any and all buildings on the property?
His wife stated that this is why he let his FFL go.
The BATF showed up, unannounced, then spent the next 30 minutes looking around the garage, the yard tool shed and the house.
They apparently just looked around and didn't open anything or ask for anything to be opened.
The "event" was a bit un-nerving to his wife because, he was out of town and she couldn't get hold of him until after they left.
So if you decide to go legal with the cleaning business and get a FFL, remember the BATF can search the place of business and any building on the property, any time they want.
Contact the BATF with your question to stay out of trouble.
You could probably get away with the cleaning business if kept word of mouth customers.
I won't go into the liability you might face if a firearm, you worked on, goes boom.