Definitely not too late for D3, D2, and even some D1s. My son had several schools talking with him during his junior/senior summer from all levels. As the process wore on, it really became apparent where the best fit was (academically and athletically) and he committed to a D3. (We just had a senior on our HS team commit to a D2 in the past week.)
I will say, what he found with the D1s that were talking with him was that they were pretty much rounding out their rosters at that point and it became apparent that they "liked" him, but didn't "love" him, which equated to slim/no chance of getting much playing time.
I also wouldn't throw all my eggs in the new travel team basket. If you are really interested in getting seen you may want to consider some individual showcases. My son went to a couple Top Gun and All Star Baseball Academy showcases (these were cheaper and were very well attended). He got quite a bit of interest from these, and ultimately committed to a school he got in front of at one of these showcases.
In terms of development, you are at the point where you need to take it upon yourself to get better/stronger. Don't rely on your HS coaches.
As for your senior season, definitely don't give it up. Take it for what it is and have fun with it. You only get four years to do it and then it is over. I can almost guarantee you, if you skip it you will regret it as you get older. As you said, you don't know what the future holds for you in terms of college baseball....do you really want to pass on what could be your last year of playing?