I think the situation is pretty clear - loose table, everyone limps to the hero in the BB. That's confirmation that it's a loose table.
It also means there's 8 x 400 already in the pot - $3200. This is already going to be a big pot; you do not have and will not have much control over the size of the pot.
I don't like the pre-flop bet. I prefer to let it go to flop without betting, and hope to flop solid, or get out cheap. If I'm going to bet to try to take it down, the bet has to be more than $1800, which is only half the pot. It doesn't matter how many BB it is; it matters how big it is relative to the pot. (Also, for reference, the raise was not 3.5xBB. The Big Blind is $400, which is what everyone already called - Hero said he made it $1800, which is a raise of $1400, 3.5 BB.)
When hero raised $1400 to make it $1800, there's now $4600 in the pot. (8 X 400, + 1400).
Remember, UTG called $400 with only $600 in the pot - calling 66%, or 2/3 of the pot. A hand that does that is also likely to call another $1400 to get at $4600. That's only calling 30%, less than 1/3 of the pot. And if UTG calls, everyone else is getting better odds than their first call, as well. The only reason they fold is because they're afraid of playing a big pot, or they're hoping others knock each other out. I agree with the others in the thread who are surprised there were so many folders - I'd have expected more than two to stay in for the flop, especially at a loose table.
Personally, I would check it, pre-flop. To take it down, I'd want to raise the pot - raise by $3200 - and I don't want to play that big with 98s.
Having made the bet - and having gotten callers - and having flopped the awesome draw - what to do?
There's now $7400 in the pot, and UTG is more likely to hold strong cards than to have hit this flop. Small blind is a bit of a mystery. We have flopped a monster draw without top pair, but we also have top pair as backup. I'd make it $7k to go, hoping to take it down this time now. AK can lay it down at this point, as can TT. But I can't let either of them hang around for free - one has plenty of overcards, and the other has me beat. I want them to give me the pot, or pay to play. I'm not going to hope for the Th or 5h to give me a straight flush... a plain straight of flush are more likely, and if the board pairs, neither is invincible.
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Ironically, if I had checked the deal and hit this flop with 7 opponents... I'd be betting a much smaller fraction of the pot, hoping to build it up with all those callers. My pot equity is much bigger than 1/8, so money in the pot is good for me, and I'm not yet in danger of really hurting my stack.