For small coops the inventory imho is big enough, you just need to organize who carrys what stack while looting. For further transportation use the storage of vehicles. This kind of logistics is imho part of the gameplay in 7d2d.
I don't play singleplayer but i could imagine that the inventory might be a little bit small there. However, there are enough mods out there that don't mess up the gui, and they also usually don't outdate within one alpha version. So atm those mods might not work for A19, but many of them will be updated soon. Since the inventory system afaik wasn't changed in A19 it would be even worth a try using a mod from A18 with A19.
And i guess you overestimate the weight system. First it might be more comfortable for looting, but imagine you want to build a base with weight-system. A block of concret then weighs 500kg and your max carry is 100kg? Second, said third time now: It's not just giving weights to items, but the system would need other mechanics implemented.