@PolyENG - Welcome to the Forum! You have done exactly the right thing up to now. You have work experience, which is valuable for engineering graduate programs, you have proven yourself in graduate courses (I presume your graduate GPA is above 3.0) and now you have to plan your next move.
The first thing is to decide if you really want a Ph.D. Your industry experience will tell you if it is worthwhile for you to dedicate the next 4 or so years to research for your career goals. Given that you decide that a Ph.D. is what you want, then apply as soon as you can. There is little value in engineering to getting a M.S. with thesis and then starting over for a Ph.D. You should take your current academic record and move into a Ph.D. program right away. Ideally, if you can stay at your present institution, all your coursework will be applicable to the Ph.D. at the university. Of course, if the research opportunities at your current institution, are not what you want, then you will need to apply elsewhere and the cycle begins in Fall. In that case, you might need to take the GRE soon and then see if there are any opportunities to enter a program in January 2020 instead of waiting until the Fall of 2020.
Alternatively, you could simply complete a coursework M.S. and then apply for a Ph.D. after that. It might stretch into 2021 if you are taking classes part time but you might be able to finish by Spring of 2020 as well.
Good luck!