Aww sis hindi ka overthinking, legitimate concern yan especially since exclusively breastfed ka pa! Honestly a few of us with BF babies went straight to open cups or those weighted straw cups (the ones na may weighted bottom so the straw reaches the milk) instead of regular bottles, kaya less risk ng tooth issues and mas parang natural ang positioning para sa kanya. Para sa cleaning, yan talaga ang trade-off, minsan I just soak the straw parts in hot water with a bit of baking soda overnight, way easier than scrubbing every time, then use a thin bottle brush or even a clean piece of cloth tied to a stick to push through kapag stubborn na residue. If you really want to use a bottle, marami na ngayon na anti-colic or orthodontic ones designed to mimic breastfeeding positioning, pero if she's never taken a bottle it might be a battle pa rin with your lo. Personally I'd lean towards the open cup route since she's 18 months na and babies at that age can usually handle it with minimal spills, pero do what feels right for your family!