Can a NON-IMMIGRANT ED PLUS visa be converted to a NON-IMMIGRANT B visa in Thailand if I get a job?

To my understanding, you cannot get conversions. But you have to take care of it at the Thai consulate outside of Thailand 🇹🇭. The sponsor has to complete the application 

Yeah. They kind of didn't think that one through, as usual. ED Plus to allow people to stay for finding a job, but then no clear process on converting that visa. In theory, it is possible to get a B / working visa in country off of any current stamp - if the tons of required documents are provided. A little easier if aiming to work for a school / university compared to a private company. Brandon gave you the detailed outline.

Converting in kingdom feels employers overwhelmingly difficult and 99% they send you out to apply initial visa.. Can't say why🤔

That depends on a lot of things.

First of all, if you are on the initial 90-day non-ED visa, then you cannot change it to anything else. The only option you have if you're on the initial 90-day visa is to extend based on education. Otherwise you need to leave Thailand without a re-entry permit to end that stay and start on something new.

If you're on an extension of stay based on education (the stamp in your passport literally says "extension of stay") then MAYBE you can convert to non-B. You would need to check with the immigration office where the company is to see if they will allow it, and you will need to check with the company to see if they can provide the paperwork required. It takes a mountain of paperwork to convert to a non-B visa in Thailand and most companies either cannot or will not provide the required paperwork because of how much it is. Most companies send their prospective employees out of Thailand to apply for a non-B visa at a Thai embassy nearby as it requires much less paperwork.

If both the immigration office of the employer agrees and the employer can produce the paperwork AND you're on an extension and not a visa, then you would need to get a letter from the school with a date that you will stop studying, and take that letter to the immigration office that issued the education extension and get the extension cancelled. Then you would apply for the non-B at the immigration office of the company.