I always tell everyone NewGrounds is great.

bluesky does allow nsfw, and i also like on it that it has a nsfw filter on it so you can toggle what you see etc.

it is currently invite only, but there seems to be a bunch more people on it and therefore more codes are being generated quicker ~ I'm liking it so far

Its probably the most mainstream outta the options ive heard ngl so I'll just have to get into it lol

bluesky does allow nsfw - and the filters applied by people are to a point some people have been merging their main content and AD/nsfw content to the one account, while others use their first invite code to make a second account/nsfw account (ill probably be doing the latter, but not straight away - so i have an invite code i can send if you need one!). either way, it is allowed. it is however, at this point in time, invite only, which def can be a little bit of a hassle and limits some growth right now, but it's probably the main growing site you'll see most people sharing links/their users for outside of twitter. you get invite codes i think like, every 10 days (might be a tiny bit less, but i know its somewhere around a week and remember someone mentioning the number of 10 days for it). and because of the amount of users joining and therefor more users generating new invite codes, it's grown and continues to grow a good bit!
itaku (itaku.ee) is nice because it allows both sfw and nsfw, to the point it has a sfw profile and a 18+/ad profile, but it's not super mainstream like you're looking for, just growing.
inkblots another place that allows sfw/nsfw with content filters like here, but it's still only growing, too.
and like jackofminds said, there's also newgrounds - which while not as mainstream anymore as much as something like twitter and tumblr- is a known name by many people and probably still has a bigger userbase than some of the growing sites, but it is known.

wish you luck regardless if you decide to try to build your audience in other places alongside here though!

Ty for the in depth answer it seems I'll just have to take the plunge with bluesky based off all the answers lol. I really appricate you listing other options n all tho :3

no problem at all! and yeah, haha. it can be a little scary to take the plunge to join it when it’s still invite-only/not entirely open yet, but on the bright side, no bots that spam your replies if you happen to mention commissions or patreon or anything like that the way twitter does lol. and you get to grow alongside the sites userbase, so it may be a little easier to grow your audience there as it grows - especially if you’re mentioning the right words so they show up in feeds people are following. it’ll take some time but so far it seems to be the promising alternative. wish you the best of luck!