Welcome to Eorzea!

Unfortunately as EveK said above, all servers are pretty much crowded at the moment due to the perfect storm of the pandemic keeping a lot of people indoors/at home, a lot of people being on holiday due to the northern hemisphere summer, and the sudden increase in former WoW players joining the game due to popular streamers trying FFXIV and.. other reasons related to WoW's developer I won't mention. Either way, it's created a massive surge in player numbers, causing pretty much all servers to have some form of congestion now.

There is the World Visit system that allows you to visit another server on your Data Centre but there are some limitations what you can do on a server that isn't your 'home' (like, you can't join a Free Company on another server). SE are bringing in a true full World Visit in the next expansion due in November that will allow you to visit any server in the game regardless of Data Centre but that won't be available for a few more months alas.

Either way, if you're happy with Siren then it would probably be better to stick with it and have your friends join you too - you can transfer your characters to another server but it's a paid service, and you cannot access it until you have logged at least a month of subscription from what I remember (I need to verify that though). But at least if you don't like Siren you will be able to move to another server without having to create a new character.