You can combat EL fluctuations by having areas of differing levels of danger. If your PCs are starting in the central Hex, have the surrounding 3-4 hexes in all directions contain EL 1-4 threats, then as they get further out have different zones (or fixed areas) that contain higher level threats.
You could have 'zones' with each zone a cluster of Hexes with threats grouped by tier (heroic, etc).
5E is pretty forgiving. PCs can hit and defeat a higher CR threat presuming they're fully rested. They may take that high level threat as an indicator they're in a dangerous area and are in over their heads.
As for upgunning low level threats, thats also pretty easy. If PCs hit an area where they're of a higher tier than the the area, give monsters in that area a slight up gunning (+2 to Saves, Attacks, Skills, +50 percent to HP, and all damage dice increase by 1 step (d4>d6>d8>d10>d12>2d8)) for each tier the PCs are higher than that of the area.