thanks to the supererogatory efforts of alecxs my backup phone is now adware-free. he went above and beyond to help me in the comments.
to eliminate vendor-installed adware without rooting phone you need SP Flash Tool, which is available (at least) for Windows and Linux; and an unofficial TWRP image available from https://romprovider.com/2018/12/blu-dash-x2-...
on my Android phone, also 6.0, I get to "critical" memory usage frequently since it only has 8 GB. and from Settings | Apps, I can move individual apps to the SD card fairly easily. here's an example.
Settings | Apps
Select SD card
You can't move apps at all without root, but you can use apk editor pro to change the storage to prefer external on other device whose storage is free ,it does not require rooting.
And there is no other way without rooting.