playing 7 handed
Villain has been loosening up since the new level of 100/200k blinds has stacks pretty shallow now. he's lost a few hands at showdown showing marginal strength

Villain has around 3.9M
we have about 3.7M

villain opens UTG 400k folds to us in BB with KJo, we think about 3 betting, but figured it would be awkward to 3! without shoving here. anyways we call

pot 1.1M
flop
Q85ccc

i check , he checks

turn 8

i lead out here 400k , he calls

pot 1.9M
river 5

i figure he is not that strong, and we are holding all the 8's and 5's (but mostly 8's, as i dont think i would lead out turn with just a 5), and we shove our remaining stack of about 2.7M. we are also holding the J of clubs...this is an alright shove?

Bluffing cant be that bad in this spot, but the question is, if its best to overbet shove or go for a smaller size like half pot. The overbet obviously put more pressure on him, but you are really only representing an 8, and if you run this line with all your hands, that missed and contain a diamond, you end up severely overbluffing. Which a good opponent might figure out and decide to make a hero call. A smaller bet can also rep a flush or even a hand like AQ, and depending on dymanics could perhaps get nearly as many folds?

I would check/fold. Villain opened UTG, checked back flop, and called turn on this board. It's very reasonable for him to have 99-JJ, AQ, KQ. With hands like AK or AJ it's quite possible he would have instead bet the flop or not called the turn. You even block the hands you want him to fold, AK and AJ. As villain I really would be shocked to call and see you with AQ or an 8 or a flush betting 400k and then 2.9M. So the big overbet looks a little suspicious, but it'll get some folds. I think a smaller bluff is terrible, even something closer to pot I could see Ace high finding the hero call. Something else to realize is you don't have a ton of 8s or 5s in your range and you do have a ton of garbage he's potentially inducing to bluff with his flop check on the three club board.

This isn't a good board to bluff at - he opened utg with 20 bb in what I understand to be the deep stages of a tourney so his range is fairly strong. On the flop he checks back - if he had say Acx he would probably bet, so his check back range is still fairly strong (maybe Qx,99-JJ,77). Yes out of the BB you can have all the 8x but he should know when he checks you're probably going to fire away and you might unload the clip with one club which makes up a ton of hands. Not a good board to bluff at, especially vs what should be a strong range.

Most of your analysis I agree with, but defending a minraise from the BB vs. an UTG open should mean we have all the 8s and 5s in our range and villain likely does not.

I'd be very careful limiting Villain's range with his checkback. There are 2 strong reasons to do so. One is that monotone boards are played very conservatively, the second is ICM. It is a big RA board for you though, so there is that...