Como dizer "Obrigado por esta parceria" em inglês

Algumas opções:

Thank you for partnering with me.
Thanks for your (continued) partnership.
We are glad to have you as part of our team.
We are happy to partner with you.
We are thrilled with our partnership with you.

Se você quiser falar de forma mais elaborada, recomendo a seguinte (muitos resultados na busca do google):

"We are glad and honoured to be partners on this journey with you and the team."

Fontes:
- textranch
- Ludwig guru