I want to use them as main infantry to replace conscripts in between early and mid game.
I know this is wrong, but any other use seems so bad compared with other tools soviet have.
I understand it's no use to spawn more than one guard.
This is completely the wrong way to use these units. Use them to back up conscripts and merge to keep them on the field longer.
You have identified the problem yourself, you need to change how they are used not ask the unit to change.
2 Guards can be very useful in cases, but I would never go more than 2. Their AT potential with two squads is actually respectable (40x4=160 damage on first volley).
Guards are some of my favorite faction infantry. They function decently in many roles (almost impossible to not find a use for them during any engagement), they also benefit from faction specific abilities (merge), and have a nice large squad size making wipes rare.
On the drop rate, that was standardized with other units. They drop items more rarely because they carry more. Stop getting them down to single men when you retreat and you will find they do not have butter fingers.