I am lucky to have an outstanding mechanic he has about a dozen ASE certifications including master auto technician and master truck technician. He would be the first to tell you the trade sucks but after 30 plus years he knows nothing else. He did try to open a legit shop after his I think 9th location ended. He's always found a shop with a free bay that type of thing and they never last. Hes done dealership work and town fleets including all the cop cars. After his big shop failed hes been too 4 more locations that I know of and now he does what he has always specialized in Limousines. Has to be the 4th or 5th limo company hes worked at and they always allow him as part of his service too work for his 200 or so private clients. I don't think hes ever made 50k in a year. The real money is in owning the shop but he quickly found out that wasn't the case for him as those 2 years I doubt he made 20k take home the shop rent alone was nearly 3k a month.
If you decide to expand and run your landscaping company then you are a business owner. You will learn what it takes to run one as you comply with all the rules and make all your obligations and if you add employees you will know what that really means.
If you go the mechanic rout you will save your money if your smart and perhaps someday own your own shop. Rent will likely be the rout you go as most small shops around here that are licenses to do auto work will cost a million dollars minimum. The big ones forget about but they allow you to make the real money in that trade which is selling used cars off the lot.
You have an important skill set that compliments landscaping. And that is being able to work on motors and that will save you thousands a year and that is a huge advantage. I do can do metal fabrication and welding but your trade would be a bigger help to my operation but you work with the skills you have.
If you get the certifications and landscaping isn't for you then you have a plan B and thats the safe rout. One of the cool things you have is the back ground to move into the heavy equipment side of the landscaping trade. That could mean future plow contracts with the town or the state or major parking lots all of it made easier because big trucks and big loaders break down and that wouldn't be an issue for you. Then there is the excavating end of this trade and again the mechanic skills are sort of a must.
This trade is no easy rout either. If I were you id step back from both and ask my self where is it I want to be in 20 years. Does money matter to me how much do I want what kind of house do I want. Do I want to give orders or take them. Answer those questions then look at both trades and figure out what would need to be the case in both trades for those goals to be reached. Perhaps being a manager at dealership will get it done if not what will do you need to be? In business for yourself? And if that answer is yes which business is easier to start and which business is most like to succeed then make your plans from there.