I can’t wait to have my first cake here

It should work if you post using a Lemmy account. However, if you use a Mastodon account it will show as “\#” which is not working as expected right now, someone has been opened an issue talking about it: https://github.com/LemmyNet/lemmy/issues/3789

Edit: After reading your comment again… oops, I think said too fast.

The situation above only applies when you try to post to Lemmy Communities, and also, I just tried it by myself, hashtag in Lemmy seems won’t work.

Testing-ground: https://lemm.ee/post/5513264

Usually, the hashtags in a Lemmy posts is a hyperlink, i.e. [#hashtag](https://example.com/tags/hashtag), for example this post posted by a Mastodon account:

https://lemmy.world/post/3754328

I don’t know if hashtag (strings) works on Mastoson, because it might be processed (e.g. HTML character-escaped) when transmitting to another instance, and the testing community i posted is not federated with other instances.

Not for now, but it seems it’s coming, see feature request https://github.com/LemmyNet/lemmy/issues/317

Edit: If you need to add flairs now, just need to add a text with bracket before the title, like “[announcement] Post title”

I found another tool: https://github.com/CMahaff/lasim, from: https://lemmy.world/comment/857727, p.s. I haven’t tried it

how to get my subscriptions from lemmy.one and import it here?

Unfortunately there is no way to do that yet, but I remembered that there is an unofficial tool that let you transfer your subscriptions like you said

Edit: There isand open issue that might talking about it: https://github.com/LemmyNet/lemmy/issues/1985

Have you tried to log out then log back in again? I heard that it may because the JWK token changes or server update recently