This isn’t good advice.
Those boxes take mythic+ score and are just a mythic 0 piece. Getting into keys this late in the season will be a nightmare and if they can do keys, the boxes are basically worthless since they can just get gear from keys.
And by far the fastest source of valorstones is delves which…per my advice above…the first avenue of gear I suggested. NOTHING beats delve gearing especially as a solo and late coming player.