I also pay for Monzo Plus and use custom categories, but it’s frustrating that these aren’t available in the same way for Joint Account spending.
For example, I have a custom “Car” category. If my partner and I pay for a car repair from our Joint Account, I can’t categorise that transaction as “Car” in the same way that I can if the transaction comes from my personal account.
The category should describe what I spent the money on, regardless of which Monzo account paid for it. A car repair doesn’t stop being a car expense because it was paid for jointly.
This also makes Trends less useful because spending on exactly the same thing can end up categorised differently depending on which account was used.
As a Monzo Plus customer, I’d expect my custom categories to be usable across my Monzo accounts, or at least have the option to make selected custom categories available to a Joint Account.