Here are my thoughts. I went to tech school after high school to be an auto mech. I spent 20 some years at a gm dealer as a tech/ Forman. The last 3 as a tech I started my business then went full time. That was 8 years ago.I will say that a diesel tech working for a fleet will be a more stable environment than an automotive tech.
Your schedule once out of school will allow you to have a small business.(do both)
Pros of you current job
Steady work predictable schedule. If you experience drought trucks still need to be repaired lawns don't grow.
Medical benefits ( I only paid about 4k per year for a family plan, self employed I pay over 14k)
Retirement plan ( if they contribute or offer one put in as much as you can while you are young, it doesn't seem important now but in 20 years you will be glad you did)
A lot of options job wise, more demand less competition( anyone with a mower is your competitor.)
I would do both take enough lawn/ landscaping to make extra money with out over doing it and sock money away. Establish yourself in life then make a decision.