Plan a local birding or sport weekend, and rent a Big White, say, the 500 f/4 I or II, mid-weight to heavy. If you don't have a suitable gimbal head, lens plate, tripod, rent those as well. Can you handle the hand held weight of the Big White? Can you walk around all day without getting tired? Do you like using a Big White on a gimbal head?
If the answer is yes, consider renting one by one the other Big Whites under consideration. By the end of it all, you may have paid 1/10 the price for one of the Big Whites in rental fees, but you will have a clear idea of which one is for you.
I get the GAS for Big Whites occasionally, specifically for the 600 f/4 L IS II. I put off the day of getting serious (I could budget for one, fortunately, given a little time) until I can easily do 15# biceps curls endurance sets and easily perform a stiff stair-climbing routine with 25# or more weights. I am a scrawny older woman, and not there yet. I also tell myself that fieldcraft matters more than having the most expensive lens - TRUE! In the meantime, I shoot with my beloved 400mm f/5.6L Small White, handheld - the lens Art Morris of Birds As Art calls his "toy lens". It is a snap to carry this around all day, and the image quality is excellent.
See how you can "beat", or at least keep at bay, Big White GAS?