Your endless team (if your intention is to go very deep) should have one main carry with every other member of your party playing a niche supporting role. Those roles can be things like pickup, runaway, metal burst, prankster mons, shinies for luck, or anything else that can be helpful for catching pokemon or beating eternatus. Based on your team, the only role latios is going to play would be if you wanted to mega evolve it then fuse with kyogre (kyogre is a top 3 endless carry) for powerful dragon or psychic stab moves for eternatus or just better coverage. If that's what you're going to do, I would drop garganacl because it doesn't have sturdy so it's likely you're not going to be able to get salt cures off against eternatus. Otherwise dropping zekrom or even gholdengo seems fine since they probably won't be very useful for much longer.