Any form of marketing, no matter how trendy and effective it is, is useless if the advertised casinos aren't able to show any good offers to attract more users to the site. Before starting marketing, make sure that the platform does have something good or unique to offer.
Any marketing that you already know OP is proven effective. It's just a matter of how properly those will be executed and as I mentioned, if users can see something good on the site, the marketing will be more effective. Aside from that, since there are lots of online casinos nowadays, marketing will be tough.
Other forums are related to sports and casinos I think will still gather visitors if properly advertised in the community. Cryptocurrency is being talked about on different platforms already and most users who have been on the internet for awhile know BTC and other coins.
I think it won't be long before the bookmakers will also start advertising on boxing forums because those are forums where bettors stick around and chat about who they are betting on.
Casinos or bookies will get good results thru advertising if the platform itself is known to the community by good reputation. Meaning, the site itself has earned its credibility throughout the years of their operation, and this is not an overnight task to begin with. Besides, it is quite easy to promote a casino, in which, everything is already in place and just needs minor fixes.
For new casinos opening up an announcement the right way here like getting a Copper account, hiring a designer to design your announcement, and getting a bounty manager to launch a signature campaign, a signature campaign is very important for a new casino, because you want recall and people's attention and what could be a better way to do that is by launching a signature campaign.
You can start with a small budget and increase it later on when you're making progress, another one is word of mouth about your casino's features and reputation
They can also run bug bounty program in order for them to further improve the services of the site. Also, if you are newly-launched casino, you may still have some glitches to address on. So much better if you open it to the community that you are accepting possible issues encountered in the site, and you are willing to pay for the corresponding reward to the person who can solve or find it.