I'm glad I came across this thread. Until I recently switched to T-Mobile (US) in February, I didn't think I needed a dualSIM phone. Then, I realized that my father (who has had medical issues the past few years) no longer "needs" a cell phone, but there are still companies that have his cell phone as the primary contact number, thus the increased need for a dualSIM.
As you had mentioned earlier, the Lumia 950 XL was an option and I was actually happy using that as a dedicated work line, in combination with a SIM slot for my father's phone number, while still being able to have my much needed microSD. That all was fine, until Microsoft somehow messed up Active Sync for their Windows 10 Mobile anniversary update! Not sure how you screw up your own software, but that lead me to a situation where I really needed to go back to Android for my secondary phone.
That's when I did a lot of digging and found that the most recent, top of the line flagship offered, that had a dualSIM + dedicated microSD slot was the Sony Xperia Z5 Premium. You can definitley find the phone for under $500, which is much less than the original overpriced value. I'm pretty sure there are a couple different models for the dualSIM variant, but the one that I went with was E6883. I've always wanted to try a Sony phone out, but always felt the price was too expensive and I didn't have a choice, as Sony phones were dedicated for GSM use. I'm glad that this one had dropped in price (from the original bloated retail price), as it's definitely a reasonable buy (brand new) for less than $500.