Don't put the extra insulin back into the bottle / pen

That's a good way to ruin the entire bottle / pen.
Sticky - Insulin Care & Syringe Info: Proper Handling, Drawing, Fine Dosing
For 0.5 units, I wouldn't draw up more than 1.5 units of insulin. Tap the syringe to float the air bubbles towards the needle. Then pull back on the plunger slightly. May have to do a few times to get those stubborn air bubbles to disappear. Then very slowly push the plunger to expel the air and extra insulin.
Some brands of insulin syringes are more prone to air bubbles than others. Try a different brand of syringes if you have more than one brand available in Italy. BD / Embecta is a big name sold nearly world-wide.
I assume you have syringes that have half unit markings? If 0.5 units seems too much, you can give 0.25 units instead. You have to eyeball it on the syringe, between the zero line and the 0.5 unit line.
Cats that have both diabetes and IBD don't need any special prescription diet. A novel protein low carb canned food or a raw or home cooked diet is ideal. My diabetic also has IBD and I feed him a commercially available freeze dried raw food in rabbit. I give canned food in rabbit for snacks. It's ok to disagree with the vet about food
Why is your cat on a renal diet, especially a dry one? Does he also have kidney issues?
An Italian member's old post on what she feeds her diabetic/IBD cat:
? - New Member, May 2, 2023: Lantus dose changes There are other food suggestions in that same thread.