I tend to agree with the Best Buy guy. I can't do a detailed comparison of all those options, but I can tell you what I do.
I use my Amazon 4k Fire Stick remote 90% of the time. It has voice control, and controls the basics of my system and all my streaming apps and DVR as well as related power and input switching and volume control. For the rest (playing music, blu-ray, etc.), I use Nevo (or any similar Harmony) to access those other functions not handled by the Fire Stick remote. But I've added IR to the Stick so that I can fully control it (minus voice) with my Nevo. My kids use their phones to control the system. The phone remote I use (theHomeRemote) does everything my Nevo can do plus my thermostats and lights. A Google Home or Alexa type solution does open the door to home automation if you think you'll ever want to go that direction.
I like the Fire Stick better than the other options you listed because it runs all my streaming apps, OTA DVR (Recast), is cheap and has voice control as well as a remote that controls my AVR volume. The others (Shield, etc.) are also good options, but more expensive and complex. Roku is also cheap and a very good streamer (I have lots of them) but can't be my OTA DVR like the Amazon system.
If you're interested in Nevo's, buy them on ebay, not Amazon. I saw one the other day for less than $30, and that was the high end C3 with RF, touchscreen, and rechargeable battery. Supplies are much tighter than they used to be since these went off the market many years ago. But I'd say the average prices I've paid were about $15 for C2's and $30 for C3's. I would never pay $120 for a C2 when an equivalent Harmony is $40. I have many, many Nevo's, enough to last me the rest of my life.
Having said all that, technology changes all the time. It's a moving target. The best solution today probably won't be the best solution a year from now. I've changed remotes and control systems many times. So just pick the thing you like best today that checks all your boxes, but don't spend too much.