wondering if there's a startup opportunity for lab testing peptide batches somehow

doubt current supplement labs would be up to the task of certifying things injectable

very risky to bypass gut digestion safety in the human body with unknowns

I would expect whomever does the certification for insulin and GLP's would be up for it. existing peptides on the market not counting all the peptides people eat daily in foods

It already exists. There's no reason anyone would know if they never ventured into the backwaters of PEDs for sport, but third-party labs are out there which will send you back a full analysis of any sample you send it, and reputable sellers anyone bothers to actually buy from will reimburse the testing expense provided you publish the results on any number of web forums that exist for the purpose of vetting these sellers on quality and purity of their products. It's nothing like buying heroin from the street or at least it hasn't been since 20 years ago.

Also US MedSpas and other boutique pseudo not-quite-medical clinics will already inject you with just about any peptide other than insulin as well as NAD+ if you feel you need it for some reason. You don't, but they'll still do it.