Walk-in curb-less if possible will probably be the bottom end of your previous quotes.

Custom (size/ curb height / drain location) Corian shower pans at grifform.com

Solid surface walls from your local fabricator of Corian. Cost will be highly dependent on finished size and the shipping charge (shower pan) from manufacturer in Oregon to your location. The advantages; ease of install (quick to do) and it will never leak. I've seen 40 year-old Corian showers that look brand new.

Please remember weight bearing grabs bars appropriately placed for going in and especially for when coming out of the shower (wet feet) vs the non-weight bearing handle you've got on the current glass door.

I did something similar for about 25k. kept vanity and toilet, removed tub and shower and replaced with bigger shower with curb and a simple drywalled linen closet.

what about all the tile work on the wall and floor? if you remove the tub and make the shower larger, will you also redo any of the floor tile? or along that back wall?

Demo usually runs in the 1K range.

what kind of tile do you want? consider that cost (no way we can guess because we don't know what you want)

You'll need new glass shower doors. figure 1500-2000K

new fixtures. 500-1200

curbless showers require more labor and a diff drain. more labor is more money.

Grab bars. not expensive, but they do need to be adhered to a stud.

I'd say you're in the 7-10K range. Could be more. It all depends on where you live, what kind of residence it is, what kind of products you purchase AND the skill level of the tile person you hire.

You could get joe blow tile guy and he could prob redo everything w/a total cost of 4-5K. but it wouldn't be done correctly.

WOW-keep looking. Bff has been a wheeler for over 30 years. She has had two 'walk in showers" converted to roll in with a bench she can transfer to, hand held shower nozzle and they didn't cost 5thousand. Over all it's the labor that's the most expensive. Fixture prices are on a continuum that if you let the wrong person talk you into the wrong end you will pay through the nose for not a lot of FUNCTION difference. DH redid his entire bathroom and it wasn't 5K.

Once upon a time there was a fiberglass shower stall you couldn't turn around it here...now it's a completely walk in shower. No walls no doors. A curtain works just fine to contain the water.

Honestly what you are planning is still the same amount of demo and construction as a whole Reno. Less the cost of a vanity, new toilet and a tub you will likely still spend at least 35k.

Where are you located?

You don't want someone who is not a tile pro because if you do that, you'll most likely wind up with a poorly waterproofed shower that will wind up leaking in a year or two. Depending on where you are, the cost could be anywhere from $10,000 to more livelily $20,000+