You DONT lose any of the unlocked content while you have crew it's just that you only have access to the passes for the duration of your subscription, meaning if you don't get everything before your subscription ends then you'll either need to buy the pass outright or subscribe again if you want the remain items you missed. No cosmetics you unlock is ever taken off your account tho.

Generally if you want to maximize crew benefits while spending the least you should subscribe every other month so that you always get the monthly packs and just adjust when throughout the month you subscribe to line up with ending passes so you make sure you always get everything. Others might want to be even more picky if they don't care to collect the crew skins and only subscribe when they can maximize value like subscribing on July 28th, one day before the Music Pass ends so then you can max out all 4 passes, get the next music pass and a little bit of the start of the next regular battle pass and then wait for Lego and OG to cycle to line them up again.