The problem is the entire structure of the marine army doesn't fit the TT and fluff at the same time. You don't send 100 space marines to go kill an army of around 250 guardsmen or similar amounts of troops, and taking obscene casualties in that battle would be humiliating. Space Marines need to become purely Elite choices, with their main infantry becoming serfs.
Ws5_Bs5_S5_T5_I4_A2_LD8 with a 3+ invul save.
Terminators have 1+ to toughness and strength. They have a 2+ Invul save and 5+ FNP.
In exchange, space marine units have their points cost tripled and would fill an elite choice in the old FOC choice, and an army will only have like, thirty space marines max.
The main unit of Space Marine armies becomes Serfs. Serfs have stats similar to Veteran Guardsmen, but don't have heavy weapon teams. The average space marine army will have three squads of marines max, with the rest being filled out by vehicles or serfs.