Suggestions: air-gapped signing and broadcasting of transactions

Hello Bitcoiners,
I'm hoping to have some feedback and suggestions on how to go about this.

The setup:
USB-Drive with Tails OS installed. I created a persistent partition on it. Within the persistent partition, I save the wallet file and the AppImage for the latest Electrum wallet (currently 4.5.2). Wallet seed is also safely stored on metal plates.

On my Android phone, I have the Sentinel app version 5.0.1. I have imported my public key, so I can see all my transactions and current amount of BTC. Receiving BTC is easy, I display an address directly within Sentinel and that's it. However, I haven't worked out how to send BTC yet.

What I do:
Within Sentinel, I select the recipient for my BTC transaction, the amount and I select from which collection (I have just one). After setting the transaction fees, I click on "compose unsigned transaction". Eventually, I have an animated QR-code (which changes a few times per second).

I boot from Tails OS, open up Electrum etc. and there I choose to load a transaction from QR-code. I scan the QR-code from my phone and Electrum throws me an error saying:

Electrum was unable to parse your transaction: SerializationError('Failed to recognise tx encoding, or to parse transaction, raw: ....')

(EDIT: further specified the error message)

Unfortunately, I couldn't find a solution on the web so far. I've seen that in the tutorial on Electrum's website here, they do both the composing and signing on Electrum wallets. However, in that scenario, transaction files are being copied from one computer to the other.

I tried to setup a watch-only wallet on the Android version of Electrum but there I didn't find a feature to compose unsigned transactions.

Could it be that Sentinel can do this only in conjunction with the Samourai wallet only? In other words, the QR-codes are not compatible?

My goal would be to compose an unsigned transaction on my phone, scan the QR-Code within the Electrum on Tails OS, sign the transaction there and then scan the signed transaction to broadcast it using Sentinel.

Any suggestions or maybe recommendations on better ways to do this would be appreciated. I know things like coldcard exist, I just would like to not get any additional hardware at the moment.

Thanks.