This is rather a WiX challenge not a Wix#.
is the setting that triggers the UAC prompt. As far as I know MSI doesn't consider scenario when per-user install requires elevation.
Of course you can either build with Wix# your custom bootstrapper app (BA) that requires elevation while keeping per-user install model. This will achieve the desired behavior. You can also check in the first install as a Launch condition or in a OnLoad event
if the user is an admin and fail the install with an instruction to restart the install as an admin.
However quite possible there is a work around within MSI/WiX that I am just not aware of. You can post this very same question to the Wix#
where currently Wix# is hosted. GitHub may have a wider WiX audience so there is a chance that someone will offer a hint.