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I have just tested with Eth Goerli Testnet. With Metamask, you can use any of your derived account, but it appears that in Suite, you can only generate a new one if you have used the previous.
My test:
I used Account #3 in Metamask and got TestETH coins from a Faucet. I did not see this address in Suite. Then I sent ETH from Metamask from ‚Trezor Account #3‘ to ‚Trezor Account #1‘. The incoming coins showed up in Suite in Account #1. Then it was possible in Trezor Suite to add a new Eth Goerli account. When I did this, along with Account #2, Account #3 also showed up.
I guess you could try to add funds to your Account #1 so you can add an other account, and maybe the other accounts also show up, but a dev might know a workaround, or they could just remove the limitation of adding new accounts.