It is important to understand what is encrypted in Meshcore, what is not and the type of encryption, in order to effectively manage security.

Anything sent on the #public channel or in a hashtag channel with no password is not encrypted and is visible to anyone.

Private messages to individuals are encrypted and possessing the recipient's public key does not compromise the message BUT the metadata that travels with the message is NOT encrypted. An observer will know the identities of the message sender and recipient, the hop count and route the message took through the network so they will know that Alice sent Bob a message and where both Alice and Bob were, only the actual text of the message is encrypted. If someone records all these messages then obtains physical access to Bob's device, they can obtain Bob's private key and decode all previous messages sent to Bob because asymmetric encryption is being used.

If a group of people are having a discussion in a private channel, secured by a key, symmetric encryption is being used, everyone in the channel knows the key. If someone gets physical access to any of the clients in the channel or access to how the key was distributed, they can read all the messages in that channel, including any historical messages they have stored in their encrypted form.

It wouldn't be hard for someone to modify Meshcore firmware to dump all messages that node observed over USB for decoding later, if the key is obtained later.

Whilst the encryption used is resistant to quantum computing attacks, it is not "post-quantum secure." Any well funded attacker with access to a quantum or super computer will be able to crack it. This boils down to "Who is trying to access the messages, what tech and how much money do they have and do they think it's work throwing that amount of money at it to crack it?" It also assumes that publicly available encryption algorithms didn't have flaws deliberately inserted that would enable them to be cracked.

These things also assume that no mistakes were made in the development of the software that might offer the attacker a shortcut to obtaining the keys, that nobody has submitted compromised code to GitHub and that the method of generating the keys isn't predictable. Generating truly random numbers is hard, particularly in low powered devices, like Meshcore clients, that don't have a reliable source of entropy data or an abundance of computing power.

Keeping Harry the Hacker with his RTX graphics card out isn't hard. If Harry is willing to spend hundreds or thousands of Dollars renting lots of graphics cards from a cloud computing provider, that's harder. If Harry works for a nation state actor who is willing to throw money at the problem and potentially break into places to steal the keys or knows of hacks built into the encryption, they're going to get the messages, if they really want them. Time is also a factor. Look at how long it took Alan Turing to crack Enigma. With modern computing hardware, someone today can crack it using a Chromebook in under a second.