Most people that use a 500/4 through 600/4 are birders and they generally shoot very regularly. As often as weekly, and generally at least once a month.
I get the sense that you've owned this lens for awhile now but you only use it very sporadically, during relatively short trips.
I think you will find that the skills needed to master this lens, and to start to understand sharpness issues as best any of us can, are acquired through regular use, where technique and gear can be steadily tweaked and progress monitored.
That may or may not fit well with your plans and usage, but I think it is something to consider. It takes more than a few days in the back yard to master it. You might consider doing some regular birding, for the practice if nothing else. Or sports of course, if that works well. Anything that gets you out in the real world with that lens.