Wow, time to venture out a bit on your own!
First off the machine.... My favorite machine I use on projects I really want precise and perfect is a 1956 singer, your ( mentor) would probably have a heart attack!
You can keep the machine you are comfortable with for the rest of your life if you want to.... There is no reason to consider upgrading/ spending money on a new one if the one you have works for you! My daughter has been creating quilts, clothes, costumes, crafts on a little $89 wally world Brother for almost 20 years, she loves that machine ( and hates my $5000 viking)
There is No Reason to believe you are not ready for Mitered borders! I've seen many new quilters successfully attach Mitered Borders to their very first quilt, you just need to watch a couple tutorials/ videos, once you understand the process miters are Not Difficult! If that is the look that would enhance your quilt go for it, some quilts ( need) mitered borders.
Generally if mitering you sew your multiples together first, if making straight borders you add them one at a time.
It's time for you to jump in and finish one of your quilts (I would actually do one with mitered borders) then when its done, Show it off! To your ( quilty sheriff) prove her wrong!
Many, many longarm quilters use polyester thread! Cotton tends to break A Lot! With the high speed machines! Its pretty common practice, even the most spectacular big famous show winning quilts have been quilted with polyester thread!!!
There are so many fabulous battings available now, it is just ( sad) to limit yourself to only one and never try anything else. I purchase Dream Poly by the bolt because that is the one most of my customers request most often. I do have cottons, wools and other blends but 9 out of 10 request the poly ( unless you told her your.... Friend....would have no idea what batting you used)
Read packages, read info on line, choose a different batting for every project for a while & see what YOU like!
When someone starts taking the fun out of what you are doing its time to step back.
sometimes folks with good intentions can become horrendously stiffling.