Reread the relevant Misadventure and the Dread is indeed Ascended Mode exclusive, so there should be no real downside to going to FoA content asap to get your hands on the altar. Depends a lot on how punishing the downsides of the altar end up being, the one we saw is double monster health and +15% OA/DA, which will certainly slow progression down unless you’re already well equipped. Another factor will be what the hostile faction for the Kurn ends up being. If it’s Ch’thonians and not some new faction, you’ll want to make contact with the Kurn ASAP to get that rep going. So, there could be a number of incentives to do FoA before the Necropolis and the XP rewards might end up being appropriate. There is no real upside to going to FG early unless you want a specific MI from there. I think I did one or two playthroughs where I went there first and the XP gains were already kinda unattractive compared to what I could get going for the Cronley quests.
As a side note, I realised I’ve been thinking about Ascended affixes wrong the whole time. I thought they were generally available but it seems they might only drop on normal items, not MIs.