The 1939 campaign is tough, because you've got very little time to rectify the sorry state of the Soviet army before Germany decides to pay a visit. As said, officer ratio is critical, at least up to around 100%, although it's still beneficial up to 140%. You've got to focus your limited Leadership on the things that really count, which mean Infantry weapons and Doctrines (the Infantry Warfare and Mass Assault doctrines), Armor techs and doctrines, and a few other critical techs. Germany can research just about everything to current levels.
Note that AT brigades are useful if they can pierce the opponent's armor, otherwise they're a waste of a brigade slot. Keep your AT piercing as high as you can afford to research or else don't even bother with AT guns, because except for that very important Piercing stat, almost anything else would be more effective in its place. The Soft Attack value of AT is negligible, and MOST divisions, even many armored divisions, are more soft than hard, meaning that Artillery or another Infantry brigade would be more effective if Piercing didn't enter the picture. The SU fields enormous numbers of AT brigades by default, so you need to make the best use of them by keeping their Piercing up to date, or even ahead of it if you can spare the Leadership.
I'm used to playing Axis minors in many campaigns, where there's insufficient Leadership to cover all but the absolute basics. You end up doing a couple of things well and simply ignoring the rest. The SU has a bit more than that to work with, but the same principle applies. Do well what you absolutely need to do, and then use the couple of points of research left to cover everything else as best you can.
You don't need a lot of points into Diplomacy (0.2 or so should be enough, after the first few months), not a lot for spies (1 point or a lot less, after an initial heavy surge to clear out enemy agents from your capital). The rest can be divided up between Officer Ratio and Research at whatever ratio you decide, but a measly 2-3 points into Officer Ratio just won't cut it.