Strictly speaking if you have a functioning bathroom with no leaks (even if it’s 70 years old) their is no requirement for the Strata to update to the latest building standards this is only required for new builds or refurbishments.
Sounds to me like you want to refurbish the bathroom and get the Strata to pay most of the cost. I believe a compromise is called for like Whale says. Strata redo the waterproofing and you pay for the installation of new bath and tiles.
I believe Strata would be within their rights to refuse to do anything and insist you pay for the refurbishment in total as well as a bylaw making you responsible for the future repair and maintenance of said waterproofing.
How have other within the complex handled this and are their any Special Bylaws registered for previous upgrades by others.