Theres a third option

Come out to them, but double downnon the spy bullshit.

Be late to everything because you had to be sure you werent tailed. Check corners with a mirror. Keep your hormone supplies in an anal tool kit. Put dye bombs in a decoy bag for your mom to search. Depattern the family pets. Dose everyone with LSD. Poison your umbrella. Change wigs and outfits every time youre in a room alone. Speak in code. Tell them you cant even tell them the number of times youve saved their way of life¹. Blackmail and terrorize strangers into being assets; figure out what for later. Find an arms dealer you like. Answer every question with 'i can neither confirm nor deny', 'im not at liberty to say' or 'no comment' so non-answers dont become answers. Learn to cheat at cards. Overthrow a democracy for basically no reason.

¹it's zero, but you can't tell them that!

my mom always goes through my stuff

this is abuse. please consider prioritising your own needs and either set some proper boundaries with your parents in a very direct and frank way or don't visit them if this is the kind of shit you have to put up with. you deserve better. my heart goes out to you, it makes me so sad to read posts like this. you have to stand up for yourself even if it's going to damage the relationship, this dynamic is not tenable in the long term. you don't owe them these kinds of accommodations at the cost of your happiness. please consider your own well-being here.

What I've come up with so far is to pre-prepare supplies, put them in a padded metal box. Then once I get to my parents, I bury them in the ground somewhere in whatever woods they have nearby.

I don't have any advice but there has to be an easier way, either just coming out or not visiting them. Where are you even gonna get a shovel from? "Oh hey dad can I get in the shed, I need to borrow a shovel to do something not suspicious in the woods"

Double floor suitcase with hidden compartment maybe

Are you thinking like, japanese sarashi? I've done it and it works okay ish. It's really warm and can be somewhat noticable if you don't do it right.

i'm unsure what kind of cloth bandages you're talking about but if your talking about ace bandages, those are very dangerous to use as a chest binder and i would really recommend against doing so especially since it sounds like you'd be using it for extended periods of time. a much safer option that you may have better luck with is trans tape or something similar if you still have time to order some before your visit?

usually cis people really don't notice and won't say anything even if you don't bind though. in my experience, after i got top surgery i never got one comment from anyone that i wasn't out to in my family. i think it's cause it would be awkward on their part to say something or they just really don't notice those things lol.

and for hiding your hrt supplies, maybe some kind of small lockbox with a key? then if they find it they wouldn't be able to open it and you can keep the key on your person so it's safe.

instead of hiding your supplies could you trust them to a friend or something? you don't need to answer ofc but you may wanna consider it just to avoid the risk of temperature-related degradation/contamination of some kind

but also

my mom always goes through my stuff

this would have me so heated jesus. "hey since we don't believe in privacy, can i have your bank info? got any funny sex toys? can i see your taxes?"

When you mention bandages, are you talking about using KT tape? It seems like that works pretty well, although I haven't tried it personally.

There are also compression undershirts for cis men with gynecomastia, which I imagine are accommodating of a larger ribcage.

Another consideration is, do you actually have to go visit your parents, or do you just feel obligated to? Do you have to stay with them, or could you stay in a hotel instead?

For how long will you be visiting them? If you're going to be there for multiple weeks, pre-preparing syringes probably isn't a viable plan.

I don't have advice about binding, but as for your HRT supplies, is the privacy situation really so bad you can't just put it in a drawer? putting it out in the wilderness seems unsanitary, and like too big of a hassle. you could also put it in a cardboard box for something else nobody would think to check, like a shoebox or a box for an old console or something