For what do I have the battery capacity when I cannot use it? I often return before 30% but if I need it I also return under 20%.
You have to think differently. You don't actually have that 30%. That last 30% isn't fuel you have that is being wasted, that is what the battery needs to have left behind to remain healthy simply based on the composition of a Lipo battery. You need to stop thinking of it as power that you're not getting to use when you should be able to, and see it as the battery's equivalent of the Empty mark.
Look at your power as being that top 70% only, and treat 40% as getting low, and 30% as empty, and only go below it in truly extenuating circumstances.
As for biting the bullet and buying such an expensive item, all I can say is JUST DO IT!!!
I put it off for so long for exactly the same reasons as you, and I'm so very glad that my hand got pushed by an aerial photography company wanting to give me work, but needed me to have an Inspire as the minimum. It meant I had to upgrade, and I've been so glad I did, because my previous unit was a P2 in FPV setup, so no app, no flight records, no camera control in flight, and I thought I loved it, but it took all of one flight on the Inspire to cause me to switch my loyalties immediately to it. I've not flown the P2 ever since.