The master pattern option would still require you to get a pattern with some of the design features for this particular leotard because of the princess line styling. In theory you can hack a base pattern to include princess seams but I've not attempted that myself yet and is why I wasn't recommending it for this current leotard. That means you'll still be looking for a pattern and, although there's some expense I know so many ballet makers really swear by Dani's tutu patterns.
I have this Kwik Sew pattern, it's the one I used when making the two lyrical dresses. It's in women's sizes and at the time (2 years ago believe it or not!) if you remember I had issues with it being too low at the neckline for my DD who was only 13 at the time and I had a lot of faff to sort that out. She's now got her womanly shape so it would probably be o.k now?
Just wondering if the empire seam is a problem as regards to using it as a master?
It could do you as a master for leotards with straps but, if you did want to do leotard styles with shoulders it won't help with that. Because you're doing ballet costumes rather than ballroom & latin like me, you may find you make more of this style - the only thing that might be a problem is the empire line. I say this because it's probably not a straight cut and the pattern maker may have used the seam to create shape by effectively using it as a dart (as princess seams do). That means you can't just cut & paste the two edges together to make an unseamed leotard pattern.
For my DGD I originally used
Kwik Sew 3661 which gave me a fitted blank canvas to hack and restyle into a range of different styles. Your requirements are a little different for ballet as you're not making the same skirts as me or attaching a dress over a leotard base but you may decide that 3 classic patterns would give you lots of options at your stage of sewing:
1. scoop neck leotard
2. strap top leotard
3. princess line leotard
Have I got this right?
Make it up, put it on (inside out?), pin to fit, sew, take apart at seams, trace, draw on the pattern any design changes, cut out from that and remake?
That's about it and make sure you mark your retraced pattern as the fitted version with a few notes on what alterations you made (if any). Your test garment can be remodelled and reused if there aren't too many alterations (won't be any good if you've had to dart out a sway back but other alterations may leave it perfectly usable). You keep that tracing as your 'master pattern' and if you want to change the style you just trace it again to be the basis of your new style and draw your style lines on it and trace other bits from it adding seam allowances as necessary.
You can see in the attached photos one of the trace-offs from my master pattern and the lines that I drew on it for creating a lace yoke and two different back necklines (the pattern was cut out with one to use with a cowl back and then I'd drawn on another for a plunge back. The smaller paper pieces are the separate pattern pieces traced off with extended overlap sections for the construction process.