I think as long as you dont copy the exact 39 amino acid structure of Retatrutide, and dont use the word "Retatrutide" to describe it, you'll be outside the lawsuit targets of Eli Lilly. You'd want to improve the structure in some way, possibly by reducing the GI effects, as an example:

retatrutide sequence: YAQGTFTSDYSILLDKKAQAAFIEYLLEGGPSSGAPPPS

modified sequence to reduce GI symptoms: YAQGTFTSDYSILLDKKAHAAFIEYLLEGGPSSGAPPPS

The difference is in spot 18, where it swaps the alanine with a histidine, which may reduce GI effects/nausea.

reference: https://doi.org/10.7554/elife.68719

Another improvement might be to add amylin/calcitonin receptor agonist to the sequence (similar to cagrilintide). (reference: https://www.nejm.org/doi/full/10.1056/NEJMoa2502081)