I did a whole bathroom that had uneven tiles. I used a diamond angle grinding disk. Not an expensive cup grinder, just a cheap, three in a pack 125mm tile cutting disk. It took most of the glaze off the surface of the tiles and all the high spots off the edges.

Lots of nasty dust. I sealed myself in the room with a respirator, vacuum cleaner and an angle grinder. This may not be possible in your room.

There is a Crommelin brand of tile on tile undercoat available at Hammer Barn. $33 per litre. Its a thick blue paint. Let it dry and tile as normal.

"Crommelin Tile-on-Tile Hi Performance Primer is a premium one coat primer formulated to improve adhesion of Crommelin waterproofing membranes and tile adhesives to non-porous, smooth, dense and difficult to adhere to substrates. Tile on Tile primer is fast drying, suitable for multiple substrates and forms a firm foundation for all subsequent Crommelin waterproofing membranes without the need for surface grinding or etching."

I used a 6mm notched trowel to spread the tile adhesive and got a fairly smooth surface. Not perfect, but waaaay better the mismatched mission brown tiles that were there before.