I ran 2 years D3, then transferred to D1 and ran 2 years there. I was decent in HS, but missed All State. D3 I was a contributor, but just missed the top 7 for NCAAs first year. Team did not make it 2nd year, I was 5th man. D1 walk on before Title IX limited men's rosters. Probably couldn't run D1 now with the times I ran back then, but everyone was running slower then... Traveled some, made conference teams, but not NCAA Regional.
The bad: I had 4 coaches in 4 years because turnover just happened at the end of my first year each place. Consistency was lost. The step up was tough, but not necessarily in general racing as we ran some of the same races. It was tough because D3 team had a "stud" who ran 14:40. At D1 we had 15 guys between 14:00 and 15:00. The step up was tougher academically to a top 20 in US in my major school, 25K students, and more in my major than the total at the D3 school.
The good: I got a nearly free education for 2 years on academic scholarship at D3 and got to grow some in the small pond. I was better prepared for the academic and athletic rigor of D1. NONE of my teammates from D3 ever beat me again after I transferred as I got 30s faster in the 5K training daily with a much better team. I learned a lot about training by having different coaching philosophies. I got a world class education.
Both were great experiences. Choose your school for education because chances are astronomically against making a living running. But run if you want to and can find a way.