I agree by the time you rip out tha whole tub wall and do the shower poroperly you might as well rip it all out . Any contractor witll charge extra for trying to keep tiles and work around them .There is no way to do the shower without ripping all that tile out and having the waterproofing done everywhere in the new shower space you do not just rip out some tile you do it all

Just make sure whatever tile you use for the walk-in shower has enough grip to help prevent falls.

Would you consider a fiberglass or Onyx shower system?