Hi, a few years ago I was in a shul where they had a sign up how the minhag of the shul is to not take tefillin off during kaddish - and i thought this was a good idea so I kind of accepted upon myself to take this idea on. Would I need to do a hatoras nedarim if i want to take my tefillin off during kaddish?

Shalom uvracha, You do not need to do 'hatarat nedarim', (the annulment of vows). That is because when it comes to these types of vows we rely on the hatarat nedarim said on Erev Rosh Hashanah. At that time, we cancel all of our nedarim (with a 'modaa'). See 'Halichot Shlomo', Rosh Hashanah, 1:9). Additionally, it seems that you did not completely accept it upon yourself but only partially or conditionally. In terms of the actual Halacha, see the Mishna Berura 25:56, who forbids removing the tefillin during kaddish. However, the Yalkut Yosef Hilchot (Tefillin 28:6) and She'erit Yosef (1:397) maintain that we should not admonish such an action of taking off tefillin during kaddish because the people doing that have what to rely on. In summary: you do not need to do a 'hatarat nedarim all the best!