Which particular lenses are you looking at? I have the previous version ("M") Moment 2x telephoto and anamorphic lenses. They worked great on my XS Max and 13 Pro, but they don't quite hold up to the higher res/larger sensor on my 15 Pro. I have no doubt that the updated "T" versions are up to the task, though. In my experience telephoto tends to work better on the main camera than the iPhone's telephoto camera. On iPhones with max 2-3x telephoto, the main camera with Moment 2x lens gives better results IMHO than the iPhone's built in telephoto, though obviously that changes somewhat in terms of reach with the 15 Pro Max, so I am not sure what those results would be like.
I'd not heard of Sandmarc before, but I am intrigued. The lenses generally seem to be about the same size as the Moment equivalents, though Moment doesn't have anything like that 6x lens. I'd imagine it's hard to get great results handheld.