Tiling project.

The spacers are great but you do need a tool with them. I'd try and go for a good make and make sure you can buy the spacers separately otherwise you'd be left with a bunch of useless wedges. Regards to a datam point for the bath I'd say it depends on the tiles. You don't want a small cut on the bath but neither do want to start from the bath and end up with a small cut on the tray. Neither do want small cuts at the top or the bottoms of the walls.

 

How big are the tiles?
If you can start with a full tile on top of the bath it will make things easier, but it all depends on the height from the bath to the ceiling, door frames, window, shower tray and floor.
The aim is to eliminate any small slithers of tile. It isn't always possible and you may have to compromise here and there.

 

Large format tiles are much less forgiving on uneven walls so watch out closely as your tiling to avoid any 'lipage' (where a tile sits higher or lower than surrounding tiles and the edges show)

For small dips yeah sure just build up with extra adhesive. Go over the wall first with a long straight edge and pencil mark any potential problem areas. Just add a bit more addy here and trowel flat with ur notched spreader. Visual check as you lay the tiles, much easier to correct any problems now than when bonded to the wall

What size tiles and what they made of. May well be advised to use a powdered bagged adhesive rather than a ready mix tub. Also one suitable for wet areas, top quality with all materials, including grout - don't go cheap and compromise the job

As wet areas, your gonna hear about using a cement board instead of PB, even moisture resistant 'green' . Loads of info on this forum if you search. PB and/or tile backer board requires no priming

Plaster does need priming before tiling using SBR primer (don't even suggest PVA) !!

Good luck and as above, a good tiling job needs careful planning and measuring before you even lay a single tile - invest your time at this stage and know where your cut tiles are gonna fall

 

Not wishing to give you hassle, but personally I wouldn't fit large format tiles onto plasterboard in a shower. The weight of large format tiles, particularly with thick adhesive is very close if not above the weight carrying ability of plasterboard. The slightest leak in your grout and the plasterboard will degrade, and believe me, once you have installed large-format and realised how heavy they are, you won't want them dropping off!!. https://www.tiles.org.uk/services/frequently-asked-questions/#16

Use a high quality bagged adhesive - shop around and don't buy it from e.g. the big yellow shed unless you want to pay £££££££s

If using small format, IMO you can get away with PB in a shower if tanked. Large format, I'd replace that PB with a concrete board - tilebakker, hardieboard

Every house I've lived in I've had to replace shower tiling because of poor substrate.

My tip for large format - get a long very straight edge. Get that first tile on and use the straight edge to check the adhesive depths across the wall. You can possibly get away with a slight curve in one plane, but humps and hollows are difficult. "lippage" looks awful and large-format is not forgiving. Also, if the tiles are shiny, they look bad if the reflections are all over the place!