Summoner is the class you are constantly changing weapons (pets). Its not like a Fo where you decide if you will be talis/rod/wand... You really need to perfect every one almost.
There is no perfect pet for every situation. Certain situations call for certain pets. Your subclass also impacts how much dps and how you and your pet react. Certain subclasses make certain pets more effective and others less so. Summoners have also changed a lot episode to episode, therefore, please be sure if you are looking up information that it is based off the last 6 months. Example Maron bombing groups as Su/Gu are no longer a thing. I did NOT play PSO 2 in JP but here are a few things I have picked up.
If you are using 1 pet for every scenario you are not only seriously limiting yourself, but your party too. Having your "preferred" or pet to use during normal Exploration trash mobs is fine, but be sure you are testing out different pets to learn.
For bosses with only 1-3 hit points redran doesnt sustain enough dps. Further, viola and jinga are more all rounders, both can also act as a small tank when needed. During boss fights it might seem great to keep a pet out that is hardy and difficult to kill but if you know how to recall and cycle your pets then them dieing is not an issue. If you are using Wanda you want to keep the DoT on nonstop during a boss fight., but know that your still not putting out a high amount of dps) Neither Wanda/Violet is meant for pure dps and both are instead your workhorses. They are great for mobbing and cc'ing. I would note that the personality chosen for wanda will make a huge difference if you want it for bosses or mobbing. Melon when used correctly during these situations is far superior.
Melon is good dmg for bosses. When he is sacficed synchro or popple can quickly remax your pp. Certain bosses Maron shines as the boss does so many small taps of output damage.
If you dont care about maxing dmg in every situation then my answer would probably be to rotate between only the following 3 jinga/melon/synchro.
Jewelry also impacts heavily your pets. Be sure to pick out jewelry that is ,ore specific to the pet you are using. For melon/redran think TEC CRIT. For wanda/jinga runthroughs whilst barely paying attention think auto attack.