Most SSH clients support Smartcard authentication where the private key is stored on smart cards (or yubi-keys and other similar HSM modules) and can be exposed via the industry standard PKSC#11 interfaces

please consider generic support of PKSC#11 for the desktop app, and potentially for mobile via NFC/USB interaction as mentioned in other ideas like

https://support.termius.com/forums/918310-documentation/suggestions/38522842-yubikey-lightning-usb-support-is-on-the-roadmap

https://support.termius.com/forums/215251-general/suggestions/37883017-support-for-storing-keys-on-yubikey-on-iphone

https://support.termius.com/forums/243649-ios-app/suggestions/34881820-nfc-support