I've been told that bubbles will continue to form until the epoxy is cured.
bubbles do not "form" in epoxy. they are introduced from stirring or
pouring too fast. the most common issue is bubbles coming out of the wood.
if this were my project, I would get a stiff chip brush and brush epoxy
all over the bottom and especially the sides of the cavity first - and let it cure.
this will "help" prevent any bubbles from coming out of the wood.
a porous wood such as oak, I would do two sealing coats on the wood.
so any bubbles that you do have will be from your pour and not the wood itself.
then, with the same epoxy, secure the letters in the position you want them,
and let that cure. this will prevent your letters from moving during the pour.
time consuming, yes. . . . . but, do you have the time and energy to do it again ?
you can do it - just take your time.