hahahaha okay so i won't be concerned. can you switch cards from sideboard between games or only matches
You can sideboard after each game in a best of 3 round, and inbetween rounds. Don't worry about shuffling, etc. As long as it is good enough for you and you are not cheating by looking at your cards, manaweaving, or overshuffling to slow things down, no one should care. Focus more on having fun than little hangups or any faux pas; most players won't care enough to say anything - and if they do, well advantage to you as you have them offgame and flustered before even playing
Things I would recommend:
Bring paper and pen to record your life AND your opponent's; spindown dice can roll off the correct number very easy.
Don't just reach out and grab another players cards or deck. You have the right to cut a deck and read a card, but wait for them to offer the deck for a cut (then request it if they don't offer), or ask to see a card and let them pass it to you.
When in doubt, ask for a judge, it is why they are there. It not only makes things go faster and more fairly, there is zero chance of grumbling about it afterward.
If you are going to bring a sleeved deck and sideboard, bring a few extra sleeves in case some rip.
Put a quarter, or a little toy - anything really - on top of your deck inbetween draws. It will remind you to check your upkeep before you draw so you don't miss beneficial triggers. I didn't do this at first and it definitely cost me a few games not taking a point or two of life at upkeep over a few rounds. I don't need it anymore, but it helped a lot at first to remind me.
If you have any, bring a small amount of good cards you may want to trade while you wait inbetween rounds. The more you bring, the more you have to look after so it doesn't get lost or stolen.
If your deck needs tokens, bring them. I have lost games before when I confused which facedown basic land represented a 1/1 warrior and which was a 2/2 flying drake. Also, if you use +1/+1 counters in your deck, bring some dice to keep track of how many are on each creature.
Bring something to drink and a snack, or $ to buy something at the event, its gonna be hours.
Good luck!