So my pre-health advising office required us to collect at least 4 letters to go along with their committee letter. One of mine was from my biochem TA (grad student), as I didn’t really get to know my professor in that class. It was completely fine! It may be a bit of a different case for you as you are talking about a letter from a research mentor, but that does not mean the postgrads/grads that you work with can not contribute to the letter. You can talk to them both and ask your mentor whether they would be open to having the postgrad send along some thoughts/observations/whatever to help them compose their letter. This was something that my pre-health advisors presented as an option to applicants, and although I did not end up doing it, it is a totally viable option!
As for next semester, I would highly recommend taking a science class with a smaller class size (eg not a large lecture course). I was also very worried about letters going into my senior year as I hadn’t really gotten to know any professors. I took a microbiology class with like 12 people in it and ended up getting one of my letters from that professor. Especially good if the class you take involves presentations or assignments of some sort of assignments apart from exams. I’m not the type of person to always go to office hours or go above and beyond to get to know my professors, but meeting with her a couple times in the course of preparing my presentation, asking the occasional question in class, and having a sit down meeting after asking for a letter to discuss my interests, background, experiences, motivation to go to dental school, etc was enough for me to feel confident that she would write a good LOR.
Best of luck to you! You have a good GPA and solid ECs- do well on your DAT and I think you’ll have success when you apply.