In my experience, size isn’t everything. it’s how you use it that counts. With a 100mm aperture you are not going to be making observations of 20th magnitude stars any more than I am of 23rd targets with my 400mm, which estimate is based on a roughly 3 magnitude difference in light grasp. I can perhaps manage useful observations (an accuracy of 0.1 m) down to mag 20 if I am prepared to spend enough time (hours!) on the exposure. You could manage perhaps mag 17 with a similar commitment but 15 to 16 should be entirely straightforward.
There are a hell of a lot of variables brighter than 16th magnitude. Some are so bright that they will saturate your detector and so are effectively unmeasurable.
What you really need to do, IMAO, is to start observing, learn what your equipment can do and to learn how to analyse your data. Your equipment is capable of producing research quality data far beyond that of any amateur set-up as recently 50 years ago.
I would also recommend contacting the VSS, but I’m biased. I found it extremely helpful.
Go for it!