Let's say I'm using 18 grams of grinds, in an 18 gram basket,
and stopping the shot at 36 grams of espresso, obtained in 30 seconds.

This gives me a brew ratio of 50 percent.
Let's also assume that the head space is adequate in this basket.

Now let's say I decide to down dose to 16 grams of grounds using the same basket.
I will most likely adjust the grind a bit finer to compensate for the smaller dose.

My question is, should I still produce 36 grams of espresso, thus changing the ratio,
or do I stick with the same 50% ratio, which means aiming for 32 grams of espresso, in 30 seconds?

A 2:1 or 50% brew ratio or normale would be just that 50%. So your 16g dose would be cut at 32g product. However there are no hard and fast rules on brew ratios and you can do whatever your taste buds find pleasing.

It depends a bit on why you are reducing the dose. If the cup is just too bold, keep to 50%. As far as I understand, tightening the grind will shift the taste towards the sweet. If you still aim for 36g in 30s the flow will have to be little faster, which will shift the bitter/sour balance a little towards the sour. If you keep to 50% the bitter/sour balance will stay the same. Check out jim's "espresso 101: how to adjust dose and grind setting by taste".