The short answer is, in your case, someone who’s already well down the path of losing weight, and using it “conventionally” as a weight loss treatment, no, you aren’t missing out on anything with Reta. Reta might actually set you back, requiring a much higher (and expensive) dose to match your current Tirz dose.
The people that are sometimes better off with Reta:
1: NEW GLP users who are afraid of, or have had experience of the bad side effects that can occur when first starting. Reta has a gentler “on ramp”. (And Tirz is gentler than Sema).
But you’re already beyond that point, and it’s obviously tolerable.
2: Bodybuilders, “bikini season”, and other intermittent users, generally not those suffering from obesity. Because they’re using it short term, and often trying to carve off the last bit of fat from an already lean body.
Reta’s unique glucagon mechanism can help in those scenarios* but glucagon is not nearly as useful for general weight loss. Most of the weight loss “magic” is still in the GLP and GIP hormones.
As a side note, for weird legal, not scientific reasons, Sema / Tirz / Reta peptides are limited in size to 39 amino acids making up their protein “chain”. Sema dedicates the whole chain to activating GLP, so it’s a very strong in terms of its GLP effects. Tirz has to split up the limited amino acids to GLP and GIP, so it’s weaker on GLP. Finally Reta has to divide those 39 amino acids into the 3 receptors it’s targeting, so they had to give up more on GLP to “fit” amino acids shaped to dock on Glucagon receptors.
This is one reason you often hear Reta isn’t as strong at appetite suppression as Sema or Tirz.
3. People who are weak responders to Tirz, or not getting to their goal weight after reaching the max dose. The Glucagon mechanism of Reta may allow them to lose (more) weight in a way completely different from Tirz’s GLP/GIP mechanism. So non responders might respond strongly to the Glucagon mechanism, and those who’ve “maxed out” on Tirz but still need more weight loss, can benefit from the extra weight loss Glucagon agonism allows.
*glucagon is sort of anti-insulin.
When blood sugar rises, insulin is released to signal cells (mostly muscle and fat) to absorb glucose, and lower blood sugar.
When blood sugar drops too low, glucagon is released to signal the liver to produce and release glucose, to make sure there’s enough energy for (mainly) the brain to keep working. This is an oversimplification, but the liver makes glucose using fat, starting with the fat that’s in the liver. Reta tells the liver to produce glucose even when you don’t need it.
Energy often drops when people first start a GLP, caused by the calorie deficit. Sometimes a low level depression can occur. Reta avoids some of this, because the extra glucose release it forces via glucagon maintains higher energy levels, you tend to stay more active and burn more calories (and the fat to glucose conversion burns a few), so you feel better during that sometimes difficult to adjust to starting off period.
But
1: You’re beyond this early GLP phase and your body has already adjusted to the initial shock of suddenly having lower calorie intake.
2: The easily accessible liver fat used to make glucose is already mostly gone. For people starting out, or intermittent users like bodybuilders, this reserve tends to be loaded up and quickly provides that extra energy during the startup phase.
Tirz has already cleared most of your liver fat (more slowly, “naturally” as your calorie deficit leads to slightly low blood sugar, glucagon is released to make up for that occasionally low blood glucose). When the liver is clear of fat, and glucagon is telling it to make glucose, it gets the fat elsewhere, but that’s a lot slower than using its internal fat. So that particular Reta benefit, clearing liver fat, is diminished in the case of an existing Tirz user.