Sch C Standard Mileage expense confusion and eventual Sale of Vehicles

I have sporadically used a couple of personal vehicles over many years for a little bit of self-employed business use, charity work, and medical travel. I always used the standard mileage. I thought that by using standard mileage, I was avoiding having to keep track of any depreciation stuff.

My vehicles are very old and in bad shape. I bought one in 1991 new and the other in 2004 used. Many years ago, I closed one or two businesses that I used them sporadically in. I don't remember if I converted them to personal use on my taxes or not since the were already personal vehicles, I don't remember. I might be able to find the information in old tax records.

In 2019, I started doing some odd jobs and charged one customer for my mileage on part of what I did for them. I wasn't thinking about using mileage on my vehicle since I charged them, but once I started TurboTax, I was looking at that on my taxes. Since I was paid by the customer for the mileage and they reported that payment as income to me on a 1099-MISC, I entered that as income, but am not sure if I should use the Sch C deduction for mileage, too.

And, if I do count mileage as an expense, how do I decide when I put the vehicle in service since it has been used over the years only sporadically and I only used it in 2019 for my customer for about 2 weeks, which was actually about 41% of the vehicle's total use last year on a vehicle with over 313,000 miles on it.

And, if I do count the standard mileage as a Sch C expense, how would I record the eventual sale or disposal on both my personal and business side if I haven't reported a conversion to personal use for years after closing businesses or ending business use? I'm so confused since I just use my vehicles when needed, but that is not often.

I know I could increase my refund by about $274 for 2019 by counting the mileage. But, is it okay to do that if I was actually reimbursed by my customer (which was recorded as income on a 1099-MISC), and is it worth it if I have to keep track and later record basis and do something on my personal taxes? I will not make a profit on the eventual disposal or sale, obviously.