I think your best bet would be to contact some tilers and ask their opinion or assistance.

do you have the name of the manufacturer or can you go back to the store and obtain correct identification of the ironing board then you should be able to ask the store and/or contact the manufacturer directly to obtain another copy of the installation instructions and then i suggest you ask the cabinet maker if they can make suitable adjustments

You can buy different baths for different installations. It’s hard to see but it looks like you may have a bath meant for a full island installation instead of a bath meant for an inset installation. Worth checking. Do you have the installation instructions for the bath? If not, Google insert vs island bath installation. And again the walls won’t be moving with the bath moving first unless there is an earth quake happening, the problem isn’t the plasterboard. And a bit of movement may be normal until the tiling is finished and the bath rim is fully supported assuming the bath has been installed correctly.

True the standards will be different, but as far as I know, the standards in Australia for waterproofing tubs only state what the outcome needs to be, not necessarily the exact equipment used to achieve it. I think something like this on the market would solve a problem for many people. I've just seen horrendous installations on NEW buildings that included MANY APARTMENTS especially where water clearly runs down between tub and wall and not to standard at all. I can't believe what builders get away with. The way I found out about this stuff is because I've pretty much exhausted my options. Scrolled till my eyes could scroll no more! :) Over many days. I saw a bath on Temple and Webster of all places which stated that it comes with an adhesive tile bead, and I was like, what is that?! I am now actually considering that tub, so I might not need to get a separate thing. But otherwise I was thinking another type of trim or flange attached to the tub could very well do the job just fine. I'm still very confused why island baths are more often offered as a wider bath and flange baths (and especially anti-slip ones clearly concepted to be shower baths) are narrower. I'm guessing they know what sells, but it goes against my design instincts!