Have you tried any platforms other than Positly? (I ask because you wrote "I haven’t been able to fulfill this quota through either Positly." which made it sound like you had already tried someone else).

Prolific nominally have 570 active UK Muslims (though I would expect to recruit fewer than that cited number), and 299 in the US.

SurveyMonkey claim to be able to reach 500 US Muslims.

Feel free to DM me if further suggestions would be helpful. I would also approach surveying respondents from website like Prolific a little differently to how I would approach surveying from most panels (e.g. many more checks required on the latter, but potentially higher representativeness on the former).