We don't see a full bath floor plan, does 36 inches vs 30 fit well? What size and layout is the bath?

It is a guest bath......how much storage does a guest require, as they are not living there, or you plan to have someone living there?

Six drawers on your 36, both sides 6" will be very narrow, but a 300.00 up charge if you want them is quite insignificant.

Jan updated post for plan info. just vanity and toilet space is 5’ 5”. I do expect parents when they visit to live there as they cant climb upstairs.

Sorry but it is guest bath The drawing IMO is useless since we do not see the rest of the space . If 36" all drawers is going to fit pay the extra . I never want vanity storage to be behind doors who wants to go on your knees to find all the stuff at the back. I use Ikea Godmorgon vanities for exactly that reason perfect storage. You start by figuring out what you need to store and honestly not usually as much as you think . If your parents are older the drawers IMO are a must.

You've not enough room for 36" vanity unless you crowd a toilet or lose the "drop zone" or something else. They will never move into this - the parents are short term GUESTS in a small space

If you can eek a pocket door into the bath? It would be nice to not have parents passing into a mud area for a middle of the night "wee"

You need storage? Add a linen.

CLEARLY..... a "drop zone" could go,if comfort for parents is the larger concern.

You could have a wall-mounted sink, and add free-standing storage drawers below. Our guest bath has a pedestal sink, we have a little shelving unit that holds the extra tp and some pretties most of the time, and when we have overnight guests they can put their toiletries there, and on two small shelves by the sink. You could also consider a medicine cabinet.

@bpath can you share image of your guest bath