Nothing is perfect. Your goal is to make attacks expensive as shit. Like ideally requiring dozens of hours of electron microscope time to pull off.
You can do a lot to that end though.
Use a mostly read only OS if you can, if you’re enterprising, a custom yocto build with most of the rootfs read only, otherwise a statically defined system like nix that can be readily deleted and rebuilt in minutes. There are configs out there for deleting root on every bootup and having the system automatically repopulate the filesystem. Enable secure boot if you can, it’s frankly your best line of defense. Any of these options are sufficiently weird that designing exploits for them would be a suffer fest.
Forget nail polish, fill screw holes with RTV and if you’re enterprising, the USB ports. At that point you can still get into the system but it’ll be obvious that someone scraped the shit out. You can simply swap the ports for fresh ones with a solder job if needed. If you don’t need this, use epoxy, get some all over the case seam. For the charging port, if it’s USB C PD, I’d need to reread the spec but you should be able to cut D-/D+ and the SS lines with an exacto blade right next to the connector and still be able to charge, just don’t hit the VCC, GND, and CC lines.
Finally, make a kwikset key trap and use it as either a lockbox lock for your stuff or the lock to your house. Kwikset should lull people into a false sense of insecurity but if they try to pick it they’ll suddenly be in a situation where they either need to go overt or somehow replace your lock before you get back. Keep things weird, your goal is to get an adversary, even one with infinite resources, to make ridiculous mistakes.
They’re the data carrying lines, if you cut them it’ll still charge, but no USB data can use the port.