Even Electrum mobile has a weird concept that can be considered as coin control. They allow you to freeze coins and addresses, leaving the one/ones you want to pay from unfrozen. Like I said, a weird way to implement coin control.
That exist on Electrum desktop version also, but I don't see anything wrong with that.
You can always import keys in any other wallet and send those coins, it's not like they are actually locked.
I get your concerns, but one important thing to note: even if Tangem shuts down, the wallet won't become useless. While the app itself isn't fully open source, Tangem has published their SDK on GitHub, so it's possible to build a custom app manually.
Nobody is going to do that.
Even for more popular discontinued wallets nobody ever made fork that actually works and have updates.
First, the Tangem app is open source software. This means that even if the Tangem company were to shut down, the community could audit, fork, and maintain the app independently.
I don't care if app is open source if it is crap, and if it is combioned with closed source firmware junk.
You don't mix pure water and mud expecting it stay clean.
Therefore, if Tangem company were to shut down, the cards would continue to function as intended.
No they won't, and I predict you will see that in near future.
They simply can't compete with open source alternative like Satochip and Keacard.