Hello,
Is it safe to use bridges at all? Who can read what on the server if I am using a bridge?
The Whatsapp/Signal bridge-bot thing can decrypt your chat and store them in plain text. So technically, the bridge operators can see the contents of your messages. In your case, they are probably the same people running nope.chat.
Unfortunately this is required for bridges to work across platforms.
If you are technically inclined, you may consider selfhosting your own server and bridges to fully control your data. You can also enable end-to-bridge-encryption if need be.
Second Concern: I keep getting invitations to a WhatsApp-Community I have never joined. I have declined the invitation but it keeps popping up. If I wanted to ban this chat I would have to ban the whole WhatsApp-Bot.
I believe the best way is to ban this chat from the WhatsApp client directly. Alternatively, you can try banning the room in Matrix too.