I think the answer is "no", or perhaps even "not yet", but is it possible to build SPM dependencies in Release configuration while the app target builds in Debug?
The only thread of info I've found is this GitHub issue Build host tools in release even when leaf target is building for debug · Issue #7233 · apple/swift-package-manager · GitHub which indicates it's not a supported feature. It'd be great if there was a way to always use Release mode, but opt into a Debug build for debugging purposes at will.