Hyperthyroid cat

My 13 year old male cat Harpo was just diagnosed with hyperthyroidism. His
node is palpable and his T4 is high (4.7), but his blood pressure and all
other blood work are fine, and he's not really showing any symptoms yet.
He drinks water a bit too frequently at night, but not during the day, so
I'm not really sure if he's drinking excessively or not. My hunch is that
this is the first indication that symptoms are beginning to develop.

When it's time to start treatment, I'm trying to decide between methimazole
or I-131. With the pills, I worry that we'll have to keep increasing the
dosage over time and have to do the I-131 anyway. But I worry that if I do
the I-131 now, when his T4 isn't off-the-charts high, I would risk making
him hypo-T. Is there a danger to doing the I-131 too soon?

I'm leaning towards putting him on the pills, and if they work without bad
side effects, stay with that unless the T4 keeps increasing or symptoms get
worse. Then do the I-131.

Does that sound like a good plan? Or is there a danger to staying on
methimazole for long periods of time? And does I-131 become more risky as
the cat gets older?

I obviously won't do anything without my vet's advice, but any thoughts
from the group are much appreciated. Thanks.