The simplest I found is the convert the Umbrel seed (24 words in AEZEED format) into the master private key that can be imported in any Bitcoin wallet.
This conversion is easy, use https://guggero.github.io/cryptography-toolkit/#!/aezeed, download the page on your computer and cut the internet to do the process offline: in “Decode mnemonic” section input your Umbrel 24 words. The value of master private key “HD node root key base58” can then be imported into most Bitcoin wallet like Sparrow Wallet.