You'll find no shortage of us who have experienced the same difficulties on this forum.
At least you know what you're looking for.
I really feel for the newb who doesn't know what they don't know. I've had multiple LBS staff attempt to sell me on 58's that they had in stock and "could make fit". I've usually started by stating, "o.k. Those are 175 cranks, right? Set the saddle to 890mm from BB and we'll start from there." They can't do it without a 400mm mtb seat post.
Anyhow. Best of luck with your search for test rides in the correct sizes. And, I encourage you to keep at it until you do find test rides. I thought I knew which MTB I wanted next and in what size. A demo day and riding several different bikes back to back convinced me that I was all wrong and competely changed my mind.
When you're big and have a limited market for flipping the disappointing purchase it pays to invest in your decission making process. Hence, you see a lot of brand loyalty from us big guys, once we find a geometry that works for us.
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