Freeroll MTT with prize pool of equivalent of 0.25$ in cryptocurrency. 48 players registered. 8 players in current table. Half the table is sitting out, from SB to EP. BTN was unknown to me at the time. (i actually played with him before but i'm not yet very good at getting reads on people fast)

Stacks:
LO (Hero): 36.5 BB
BTN: 29 BB

LO is dealt Q9 and raises to 2.4 BB. 2 folds. BTN reraises to 5.1 BB. LO calls.

Flop (11.7 BB): 8J3

LO shoves.

Looking back i think shoving here was a bad idea since, assuming he's not a very bad opponent, he'd be calling only with strong hands. I believe check calling/shoving would be better since then i'd get value from his bluffs. Check calling, if he doesn't bet very high would allow me to see the turn card and possibly get away if my hand doesn't improve. Check shoving would guarantee i realize full equity of this hand and prevent him from trying to bluff me in the turn altho with such a big pot size compared to stack sizes i don't know if it'd ever be a good idea to fold the turn and therefore maybe check-shoving is the better option. Or maybe i'm completely wrong and this hand not strong enough and should just check fold unless small enough bet. Thoughts?

You should definitely check the flop, donk shoving is a bad because it's still a ~2.5x overbet shove, and you'll only get called by nutted parts of his range, and fold out anything you had a chance against. While holding this hand, you also block his suited Qx and 9x combos with flush draws that he might 3-bet preflop, like AQs, KQs, QJs, 10 9s, 98s, if those are in his small sized 3-bet range (depends on the player).

Here, you can easily check shove or check call depending on his size, as you have a lot of equity and can fold out a lot of his bluffs if he is overbluffing or c-betting too wide, and you still have equity against 1 pair or overpair type holdings. For example, if you against AA, you have 42% equity, 48% against AJ with the Ad, and 48% against J 10s (diamonds). His nutted range can have JJ, you'll still have 33%, AJs (20%), and AKs (26%). He'll also have a lot of bluffs here to continue a balanced c-bet range. Basically, your draw is probably a lot stronger by equity than you might realize, so check shoving can be great because you will win through folds a lot of the time, and if he calls you are still drawing fine, he can also just check back and that caps his range at far worse hands unless he is peeling for free or trapping which is less likely with the scarier holdings you could face.

However, you could also check call depending on his bet size and his general play style. If he bets 6bb-9 into ~12bb, you can call and make the pot 24bb-27bb with 24bb or 21bb behind while still getting odds/implied odds to draw the next card and evaluate the turn. I think calling can be fine if you knew for sure he was tighter, and jamming is profitable if you know he c-bets too much or overbluffs, but given that you have no information I would judge with the bet sizing. Also, there is always a chance he just checks it back, and you an re-evaluate or steal on the turn. Lastly, check shoving can be great here too because you are 4-handed, so he can 3-bet with more safety, and can 3-bet wider than 8 handed or more people behind. Depending on other context like structure, how deep you are, and more, I think you can craft an argument for check call or check shove based on your preference and villain's, but donk shoving is a no, and check folding is certainly a no because of how much equity you have in the pot against his range.

Given that this is a freeroll and people will not be playing super nitty, I'd probably check-jam to any bet bigger than half pot, and given no reads, I might check raise or check call if the bet is unusually small, but it also depends a lot on tournament factors and how aggressive my strategy is.

stack sixes are nice for a xrai with a draw this strong -shove will be a bit over potsized and will have lots of fold-equity