C-61 is what you need to install non electrical
signs, C-45 for electric. I thought it was four years of certified experience, at least it was when I did it. The qualifications that you described are all pertaining to the design side of the
signs, not the mechanical. When you're wiring someone's building there is a large liability and I think the licensing is intended to protect the consumer. Suppose a building fire is traced to a spark from a transformer installed by an unlicensed contractor? Most liability insurance is dependent on having the proper credential. I think everyone is protected by this.
Perhaps you could put in a day a week working for an electrical
sign installer at minimum wage. You could learn their ways and log the time.
I think the suppliers of your materials can also verify your time in the business.
Have you ever worked any construction? Do you understand how a building is made when you approach it? This is some needed info when using any type of fastener. You need to understand where the stress load is and wind loads. It's all common sense, until you see the requirements by building departments to obtain a permit.
I see so many raggedy and questionably safe installations that I'd want to encourage you to always be mindful of public safety more than getting the job because of the bid.
PS. I see they have a D45 for non electrical. When I got mine that category was not there