Then it really depends on your threat model – as in how big & juicy a target you think you are, and which part of “da man” is going to come after you.
If you’re just some rando white guy who may or may not be pulled aside for a random screening, really you just have to pass a casual inspection from the border agent who probably looked at dozens of other devices that day.
In that case maybe you don’t even need a separate phone, and can use something like a private space (for Pixel phones) or Secure Folder (Samsung) to just hide specific apps from casual inspection. That will probably give you enough cover from a casual search, since almost all of your apps will still be there, and realistically used, just not Discord or whatever else you want to hide. The Pixel version suggests, but does not require, that you associate a different Google account with those particular apps; you can still just use the same account if you prefer, just with a small loss in secrecy.
Of course, they can always wrench you and make you open your private space to check for anything there, so it’s not the same as true plausible deniability… but if you’re that enticing a target for the border agent, you’ve probably got bigger problems.
The government(s) can always just directly subpoena your stuff from Google, your cell phone carrier, Discord itself, to get what they need. Mainly you just have to look less suspicious and hope you don’t get pulled aside for a “random” screening… once they decide to really dive deep, you’re not going to be able to hide much.
Maybe distract their attention with something an inexperienced traveler would carry, like a water bottle with half-frozen smoothie or a semi-wet lunch (curries, etc.) or something over the 3-3-3 rule so they lecture you on that instead of focusing on your devices?
Edit: I think you can even have a “fake” version of Discord alongside the real one in your private space. e.g. The private space one will have your real account and all the chats. The “fake” one on your main home screen can be linked to another Discord account, and just join some random gaming channels there to make it look somewhat realistic. That way, if someone looks for Discord on your phone, they’ll see the fake one as a decoy. They would have to both know about private spaces and force you to unlock it in order to access the real one.