I am a discretionary manager too, just that I am so slow that I have not caught up to it yet.Note: I'm a discretionary manager... listen to @Slow Learning Elf for the systematic side (plus he's infinitely wiser than me).
For what it’s worth, I use stop losses whenever I get involved in special situations type of stuff. The idea is that “my read was incorrect for some reason and I don’t want to be in the trade any more”. In that case, I ab initio make a decision to either have a 2-stage stop loss (cut half at loss X, cut all at Y) or or a single-stage stop loss. In all situations I usually leave a tiny (inconsequential in the context of my book) position on so I am forced to follow the trade and see the outcome.
For my systematic stuff, I do both predefined SL/TP combinations when using classification methods (it’s naturally combines with it) as well as no stop loss (use position sizing as a risk management tool) when I am using regression methods (again, that is natural). To get optimal stop loss and take profit levels, I generate synthetic data that has similar vol/skew/kurtosis to my instrument and simulate my process to find optimal parameters. Not as straightforward as it sounds but works
I understand that you are “multiplying” your data but keep the underlying statistics and distributional characteristics intact, but in my (imperfect) view it stays “curve fitting”. When you say you need more data to build models I understand and that is a good use, but the curve, stays the curve, so to speak.