Got some reasonable pics taken of the display case as a now final completed project arranged by colour for its initial display. Clearly a lot of them could be painted a lot better, but overall they do combine to make a nice, colourful display in a compact, enclosed case. The magnetic bases keep them in place so it can be moved around easily. Overall I'm really happy with how it came out as a beginner's project.
For reference the 20 chapters as as follows from back row left:
Dark Krakens, Death Spectres, Hawk Lords, Ultramarines, Dark Angels
Aurora, Mantis Warriors, Atlantian Spears, Dragonspears, Space Wolves
Executioners, Grey Knights (fake as they can't be Primaris), Minotaurs, Blood Angels, Exorcists
Celestial Lions, Imperial Fists, Mortifactors, Wolfspear, White Scars
All have either 3D printed or official transfer chapter heralds except Aurora and Blood Angels that I painted early on when I didn't have any of the others on hand. All have alternative magnetised helms except the Imperial Spear which I glued on as my very first attempt that I went back and tried to rescue. Very happy to call the project finished and to move on to painting the Grey Knights combat patrol set that I've already built and magnetised between gaps in painting this 20 marine display.
Head's up for a few more images, hope all get some time for painting and modelling over the Christmas break.