I understand that each of Ethereum addresses in Trezor represents standalone Ethereum account. But doesn't this bring problems with sharing funds between your accounts? If 100 people send me 1eth on 100 different addresses/accounts, how do I send 100eth from one account without wasting time and burning fuel 100x to merge my coins?
Ethereum is an account based, not UTXO based system. Although Ledger and Trezor offer seed-based ethereum addresses (which results in multiple addresses), Ethereum is designed in a manner that it is better to use a single address.
There is no way to send eth from multiple accounts without first consolidating it into a single account. You must take the initial payments to a single account to begin with.