Reddit mentions of LESHP Game and Trail Camera 12MP 1080P HD with Time Lapse 65ft 120° Wide Angle Infrared Night Vision 42pcs IR LEDs Waterproof IP66 2.4" LCD Screen Scouting Camera Deer Camera Digital Surveillance

It sounds like you're already installing a security system, but a couple other options -

I'd contact your local postmaster. It's illegal for people to put things without proper postage into mailboxes, which is why you get those maid service and pest control and lawn service placards in your door jam or under your mat or hanging on your door handle. They can't just put them into people's mailboxes. Drug dealers cant leave packages in your mailbox either. You calling the postal inspectors may not get you moved up to the action list, but a postmaster calling them might. There are safety issues for your carriers as well if illegal stuff is being placed in your mailbox. Who knows what could be in there, child porn, weapons, anthrax, drugs, money, explosives etc.

If there are trees or utility poles etc near the mail box, you could try setting up a game camera pointed at your mailbox, like this one - https://www.amazon.com/LESHP-Infrared-Waterproof-Scouting-Surveillance/dp/B01MTVF2N3/ref=sr_1_22

They are usually motion activated and work pretty well in the dark.
If you had several videos of someone going up to your mailbox and putting things in or taking things out that would go a long way towards attracting law enforcement's attention

Another option, and I'd clear this first with the postal inspectors- if the mailbox inside your enclosure is a standard sized box, buy a second box at a home improvement store, cut the back of it off, punch a small hole into it, get a motion activated "spy" camera for like $40-50, mount it to the back of the mailbox you cut off, and then slide that into your normal mail box, making a false back wall with a camera behind it, and then retrieve it every couple of days to charge it and get the videos off the micro sd card. Honestly though, I'd let the postal inspectors know this is what you're going to do before you do it, that may cause them to act sooner, as mailboxes once properly installed are considered federal property.

These sound like a lot of work or extra expense for something that law enforcement should be able to handle... to be honest, I'd keep contacting the Postal Inspectors every time it happens. They're the police force for the post office and AFAIK are supposed to handle stuff like this. I know they aren't using the mail stream to deliver the junk and are just putting things into mail boxes, but the mail boxes are federal property and you're not supposed to tamper with it.

If you STILL aren't getting any sort of response from authorities, call your local news stations... this could be the sort of "stranger danger, film at 11" that gives them ratings. Heck, they might even help you set up some sort of surveillance system so they can get video of some tweaked out junkie opening people's mailboxes, that's a great optic to show on TV. You see pieces like that on TV all the time, like when they talk about catalytic converters being stolen off cars in parking lots etc.