The key thing you need to know is that delayed and lost baggage is governed by the
Montreal convention and Qatar have signed up to this. You can claim up to 1311 SDR (about 1800 USD) for replacement contents of luggage as well as items purchased to keep you going until reunited with your delayed luggage.
General guidance for items purchased to keep you going is to keep costs sensible and where possible buy items that you would otherwise be comfortable paying for yourself, and keep all receipts. Then open a customer service case, no need for any emotions about how difficult it was, just set out what happened, your ticket and flight details, your PIR (property irregularity report) reference, and what you needed to buy, when, and how much, with receipts, and keep a copy of everything submitted. Qatar customer service isn't stellar in this department but they do usually get there although it may need a few chasers. YMMV. Good luck.