[Purpur]

I see that the LG G5 doesn't support adoptable storage, so that leaves two options.

Root the phone and get support for it. I've been using Link2SD on my old phone, and it worked perfectly well. But are there any good custom ROMs for LG G5? I haven't found any. I suppose one could root without flashing a custom ROM, but not sure if it's worth it.

Wait for updates. Android Nougat is just around the corner, but what are the chances this'll allow LG G5 users to install apps on the SD card?

[SpookDroid]

I don't think Nougat will change the way the SD card is handled in Marshmallow, so to do that, LG would have to enable Adoptable Storage on the phone, but if they chose not to in the first place, is likely they'll go the same route with Nougat. If you MUST have apps on the SD card (which, personally, I don't recommend since SD cards are not that reliable anyway), then rooting and applying the patch would be your best bet.

[Purpur]

If you MUST have apps on the SD card (which, personally, I don't recommend since SD cards are not that reliable anyway), then rooting and applying the patch would be your best bet.

Just found out the LG G5 can get adoptable storage without root:
Portable/Adoptable Storage - Post #5