Albert – in response to the second part of your post (para 5), you have put forward an entirely sensible proposal, but I suspect that your Strata Manager (SM) maybe having difficulties getting a decision out of your Plan’s Executive Committee, and/or is in a quandary about the concept of you arranging works on the Common Property (i.e. the shower floor and any common walls).
For a number of reasons that I won’t go into here, I’m inclined to the view that it’s better for the Owners Corporation (O/C) to arrange for the repairs to its Common Property (albeit via the SM), and for you to speak with individual tradespersons as they arrive at your Property about separately quoting the additional work that you want done, and your payment for that.
Remember that the SM works for you (as a member of the O/C), so it’s entirely appropriate for you to put pen-to-paper and (politely) put forward your proposal regarding the concurrent renovation of the bathroom, to insist on more than one quote for the components (yours & the O/C’s), and to insist upon a timely decision particularly as delays will almost certainly lead to even further damage to the Common Property (incentive).
Copy everything to the members of your Executive Committee, and follow-up with each of them personally.