Cash game player played my first tournament live at the casino. Was squeezing a lot when there were many callers to take down all the dead money and was barreling a lot of good boards and runouts for me (I wasn't over betting so maybe I needed to use bigger sizings in some spots to play this style).
I had 63s call me down triple barrel on T2687. And TT call next to act 3 ways on flop of Q53. He called a triple barrel for his whole stack during the rebuy period on 3 turn and 3 river. I bet like 4500 into 7000 on turn and put him all in for 12,000 on river.
Then I rebought and went to a new table and was playing aggro there. Lost a hand and shoved with 11bb some hands and was getting called p light. 3 bet shoved A6o as a bluff for 12000 over like 1,700 open and got called by A9s.

Seems like the players really didn't like folding for the most part preflop or postflop. Was $500 buyin too so it wasn't nothing.
What playstyle should be used before rebuys end and after rebuys end? Stick mainly for value while rebuys are up?

Starting stack was 300 big blinds, 30 min levels.

Generally you'll want to bluff less (a lot less, from the sound of it), bet bigger for value, and go for thinner value bets.

Yes to the above. The early stages of a live <$600 tournament, imo, are a place where you will want to virtually never bluff. At least where I play there are some incredibly bad players who will put the money in with all manner of nonsense.

Thing is your image matters a lot. I am an older white guy so it looks like I will be tight and not bluff much.

Early on I will raise a lot especially short handed and will not cbet flops much unless I have some equity. Even HU. When I do cbet I will bet the turn only when I have it early on and oddly sometimes when I have TP I check the turn and call the river or bet the river. So my image is a guy who bets when he has it and checks back when he doesn't.

So I bluff a lot. And rarely get called. When I do get called I hold up (check back the river) and typically I'm up against flopped sets or trips or two pair. But if I bluff the turn I will typically bluff the river unless I am blocking a lot of draws.

It is incredibly important early on to increase your stack size leaving all options open post flop. This becomes critical as the blinds go up.

If you are youngish and GTP/Solver like then your image will be Loose Aggro. So early on I would raise a lot bluffing (like you are supposed to do) until I got caught. Then at that point I would tighten up and not bluff much (I would semi-bluff with lots of outs though). You will get called by a lot of people who think they have good equity against very wide ranges. Especially if your image is that of an overbluffer.

The history is a bit thin to really know what people were calling you with, but the last hand shows how sticky players are after raising. "3 bet shoved A6o as a bluff for 12000 over like 1,700 open and got called by A9s." Not clear what the blinds were, but let's assume 600BB so the 1700 was just shy of 3BB. You three-bet all-in for 7x, which looks very bluffy to me, so if I'm opening with a reasonable range, I'm calling your bluffy-looking shove with a lot of my opening range. It's not so much about the re-buy period being open and more about my stack vs. your range.

I'm a lot like Mr. Rick -- older white guy. Tend to start level 1 very tight. Hope to show down a winner or two, fold a lot, and build an image as a straight-forward guy who will bet/call with good hands and fold to pretty much any pressure when I don't have it. This then allows me to bluff in strategic spots (in position heads up) and take down pots with small c-bets when everyone assumes I hit that ace even when I didn't. It also allows me to pick spots to hero call and 4-bet against the young agro players who I think might be bluffing -- until I get caught. Then tighten up until the next table change.

That's generally good advice against stationy players (especially in low-to-mid-stakes) your focus should be on maximizing value rather than just trying to run big bluffs.

20BB is a pretty reasonable size for a linear 3-bet shove range. Some of the best hands and bluffs will 3-bet small instead, but not all the time.