HTPC and Seamlessly are typically oxymorons. But in general best HTPC is nvidia Shield TV
Either use with Plex (it supports both server/client on same hardware)
You should be able to control it with both iphone apps, or your harmony remote using IR
https://support.myharmony.com/en-hu/harmony-experience-with-nvidia-shield
I do this exact setup except my Shield TV is just for playback. I've got a Plex server on unRAID in my office closet.
I use the Harmony Companion and it works great.
If you don't go the Android TV route you can get a mini PC that are now very affordable and powerful. The most popular being the Intel NUC. I've got an old Foxconn with AMD Brazoz (E-350) that worked very well for many years.
The most important part of an HTPC (that is used 99% for video/audio playback) is to be sure you get a videocard that can handle all of the decoding duties. Most modern CPUs can depending on whether or not they've got a built in video chip, but standalone video cards have worked better for me personally.
What software are you wanting your HTPC to run?