Is there a known limit to the amount of bitcoinZ that can be sent in a single transaction?
The reason I ask is because if I try to send 50,000 BTCZ the wallet will throw an error.
However if I send 20,000 BTCZ, it takes a while, but after about a minute or so it goes through just fine.
If anyone would like to test, I’ll generate you a new z address to test on. 
It’s not a limit of how many coins but how many UTXOs. You can send 1,000,000 coins around as long as there aren’t too many UTXOs.
If you mine to that address, you likely receive many smaller transactions. When you try to send from that address, the wallet groups all of the incoming transactions that make up the coins that will be sent. If there are too many transactions to group together, you get an error.
The solution is to funnel coins into another address with fewer transactions. This will allow you to make larger transfers as needed.
so if I take coins from a t address which has indeed been getting 30 or so coins every few minutes and move them to a different t address or a z address, then I won’t get any error?
I believe what you’re saying, and I’m going to test this out but I’m interested to know how you found this out. Is it written down somewhere?
Also, does your explanation also explain why it’s super quick to send 10 coins, but much slower to send 10,000? I always found that puzzling.
Well, more specifically, it’s a limit on the size of a transaction. Each transaction lists all vin (incoming transactions) and vouts (outgoing transactions) that tracks coins from where they’re coming from to where they’re going to. You can see this by looking at the api for any transaction. What you’ll see is that this transaction’s coins come form one transaction (probably a shielding transaction) since this is a payment made by a pool to miners – and the vout has a list of all the miners being paid.
When you have too many vins, the transaction size gets too large and will be rejected. here is another example with multiple vin and vout. As you can see, a single transaction can very quickly grow in size.
Also, yes. More coins generally means more UTXOs being processed, requiring more compute and longer time to confirm as pools will likely opt to confirm smaller transactions first depending on tx fee.