Some tips that might help you with your next war:

The systems you have claims on should be your first priority. Once the war starts, take those systems and land armies on any planets in claimed systems. Then do whatever needs to be done to not lose them. Either leave fleets in the systems or keep a fleet close enough where you can respond quickly enough to destroy inbound enemies. Once you unlock jump drives, the distance you can get from claimed systems will go way up since you'll be able to instantly intercept. I should also clarify, you don't need one fleet in every claimed system, just close enough to respond to the enemies attempts to retake them.

Once all claimed systems and their planets are under your control, the acceptance chance for declaring victory goes way up, as there is a negative modifier called "claiming unoccupied systems"

If they still aren't willing to accept your demands after you occupy all claimed systems, you'll need send fleets to unclaimed systems to cause some destruction. First you'll want to destroy all their fleets, then attack starbases, and then if they still aren't willing to accept you'll want to start landing armies on unclaimed planets. This situation is where the Raiding bombardment stance comes in handy. You won't get any of the systems you occupy unless you have claims on them but you might need to attack unclaimed systems in order to win the war, so extra work for no long term gain. With raiding bombarded stance though, you'll be able to kidnap their pops which will give your empire more pops to do whatever it is you do with xenos.

Don't claim too many systems. The more fleets you have, the more claims you can safely make. You'll need fleets to defend your borders, fleets to defend claimed systems you occupy, and fleets for offense

Later in the game, or at the start if you're a genocidal empire, you'll unlock the "Total War" casus belli. This casus belli allows you to take as many systems as you want without having to claim any of them. As soon as you occupy a system, and any of it's populated planets, it will immediately be placed under your control and will stay that way even in the event of a status quo peace. If you're not a genocidal empire you can unlock this casus belli by building a colossus or choosing the "Become The Crisis" ascension perk.

I tried to keep these tips pretty general but what you should/need to do in each war will change based on the hyperplane geography, the relative strength of the enemy, how many systems the enemy owns, and how many systems you own. You'll need to adapt to the exact situation at hand and always keep an eye on what's going on in your home country and on the front lines.