Have you been on any GLPs before? Have you already lost weight before starting it? Age, weight , height?
Assuming the answer is no to the first two questions, it sounds like you are not super sensitive to GLP's, most people would have some appetite suppression or a bit of weight loss or some nausea by 7.5mg. I would suggest sticking with tirz for now, and then you will in the end know how well it works for you or not. If you start adding in reta or swapping to reta halfway through and you start responding you will not know if tirz works for you or not. Losing weight and keeping it off is a long game and Tirz is either the best drug available ( lowest side effects ) or the second best ( on weight loss ), and knowing for sure if it works for you or not is worth another month or 2. If you have zero side effects and do not have diabetes or other serious illnesses, you can increase doses a bit faster. There is always more risk of unpleasant side effects if you do that, but upping doses 2.5mg every 2 weeks with zero side effects is not especially dangerous. The tirz dose escalation protocol is fairly slow, compared to the others, and mostly that is a good idea.
And if you get to 15mg/w and still no effects then , maybe your tirz is dodgy or swap to reta and see if it works any better. But you will then know for sure if it is worth using for you, but if you mix it up half way through you will not know at the end. A picture I saw recently from one of the studies showed about 5 to 10% had minimal weight loss from full doses over a year so it is possible, and it would really be disappointing. Also double and triple check your dilution and dosing calculations, it is so easy to be out by a factor of 10.
If you have a different batch of tirz you can try , try it and see if it is the same, I have not heard of anyone on this forum getting vials with zero drug, but it can happen. If it was from a well known vendor who has been around a while it is less likely. A random vendor on tiktok maybe not so unlikely.