Options for raw hdr merge for Lightroom mobile

So when using the Lightroom mobile‘s camera hdr feature it creates three dng exposures, merging them to one dng with extended dynamic range for editing, which works miracles with enough light.

The point where I get stuck is creating three exposures with 3 raw images (-3, 0, +3 EV) in my camera and trying to find a way to get anything close to a similar result as the in app camera hdr feature by somehow merging them into a single file with extended dynamic range and editing it in LrM.

Since my options are an iPhone 11 and a MacBook Pro from 2010 with upgraded hardware neither apps like Brackt on iOS nor programs like Affinity Photo on macOS could help creating a merged tiff or raw image that would’ve had a decent quality or would’ve been able to be imported into Lightroom Mobile for editing.

Besides of buying a new laptop or installing a more recent macOS, is there any options to create a somewhat similar raw hdr merge with camera files like raf from Fuji in order to edit within Lightroom Mobile?

Adobe‘s support website states LrM would support 32bit tiff photos, yet importing them didn’t work, probably due to large file sizes (300+mb)😆