Keeping my bitcoins safe: any flaws in my strategy?
The segwit accounts in TREZOR Web Wallet are derived using BIP49.

So far, no software wallet I know support this, unfortunately. I have heard that Electrum is working on it.

However, if you already wrote your seed to your PC or phone to some of the wallets - Electrum, Mycelium, Wallet32 - you should consider it compromised and move your coins somewhere else. Seed is good only if you don't rewrite it to your PC or mobile phone.

Note that Wallet32 is no longer maintained; since it is a regular SPV wallet, it's possible it will stop working with Segwit2x fork. Mycelium should work; as should Electrum, or its Android clone.

Also, other hardware wallets support the same seeds as TREZOR - for example, there is Ledger wallet, that uses the same seeds and the same logic for accounts. So you can rewrite your seed there and you will have access to your coins.

Because I am an employee of TREZOR, I will tell you that it's more wasy to use than Ledger, but I am obviously biased :) but if you have the seed, you can also use Ledger with the same seed as TREZOR.