Thank you for your RSVP. We are very pleased that you plan to attend and look forward to seeing you then.

Support #1, wobei ich den Gebrauch von RSVP so nicht kenne und mit 'Thank you for registering ... ' arbeiten würde.
'arrival' aus #2 ist zu eingegrenzt auf die Ankunft.

Is the 'Tagung' a conference or symposium for which the invitees have to register? If that's the case, I'd more likely say "Thank you for your registration. We look forward to your attendance." Otherwise, I think I'd change #2 to "Thank you for confirming your attendance. We look forward to welcoming you to the (event? conference? meeting?)" Or if you think it's OK to be more open and friendly then the second part of #1 is good: "We look forward to seeing you."

Agree with #1 but I might say "...are able to attend" instead of "...plan to attend."