First of all, welcome back
Secondly, you will have to find a local group in order to observe their meta in order to understand where to go*
However, where to resume boils down to three options:
1. retain your single-dex mono build, and win with a serious curveball. Very few armies besides maybe tau or another
IG army can even hope to gun through 200 dudes in a single game, and that's before you take into account the tanks.
Guard shooty blobs are still extremely viable. Pair with many many heavy weapon squads, and add munitorum priests to the blobs to make them fearless, and win by literally making your opponent unable to position his models and drowing things in flashlights.
2. Formations and allies are a thing. Ally in for the gimmicks other armies get. You can, as Scotsman said, use the Blood Angels Flesh Tearers Strikeforce from the exterminatus campaign book to get access to 6
FA slots, for the relatively low tax of one
HQ & one Troop option.
This will allow you to drop Meltavets or Plasmavets in Astartes grade style, all the while your tide of regular dudes comes crashing closer. Or, take Mephiston and make a Leman Russ Battle Tank squadron invisible. Or ally in something bigger, like an Imperial Knight, because who doesn't love big mechs with battle cannons and a sword larger than a small tank.
3. Psychic abuse. The psychic phase is what literally breaks this game, especially when done on the
MSU level. This is done best through the inquisition. 175 points will generate you 13+
D6 Warp Charge (the new 'psychic points') each and every turn.
You can use each one of these psykers to do something different, from making your own units invisible or giving an entire squad twin-linked (
FRFSRF I hear you think?), to throwing fireballs and tearing a portal to the warp open within enemy battle tanks.
And that is just staying with the Imperium's Finest, you could just as easily pick up a new army, and make it rain Crisis Suits (Farsight Enclaves have their own codex supplement now), run a wall of wraithguard (Iyanden Eldar supplement), or run the indestructible robots, which can, as you so rightly say, gain up to 5 saves on a single dude.
The options in 7th are pretty much endless.
*For the record, I feel sorry for the headache you're about to get.