I've been using Stretch about one year before it becomes "stable" (same for Wheezy & Jessie), and quite used to Debian for about let's say .. more than 13 years ?
It's a very long time I did not break anything without any solution (even don't remember).
Testing and SID currently have plasma 5.10, which probably means Buster will use plasma 5.10 when stable.
I already have Buster (+ very likely some pieces of SID) installed for some time on a separate partition (even on 2 PCs), and investigate (at least for test) how to have a future stable Buster basis with a bit more rolling plasma on it (> 5.11).
For example , Okular 4.16 in SID still uses qt4 libraries, and 4.17 (only in experimental) only qt5.
I try to prepare something without a mix of qt4 + qt5.
Once more, this is investigation for me, without any risk at all (currently on Stretch as daily desktop).
I've just launched Netrunner in live mode for the moment to see what it is.
A bit stuck for the moment with the netrunner "gpg server key" requested by apt { edit:
solved }