You can definitely do that yourself.
I have 10 rental apartments. The bathrooms have been renovated once but not that great. Certainly simple standard with white tiles. I have now had 4 bathrooms done. First, the sanitary specialist company came. New concealed cisterns with wall-hung toilets, laying pipes. Washing machine connection with water + drain, costs about 700 euros per bathroom.
I attached the washbasins myself after the tiler. Italian quality tiles, calibrated, I bought as a special batch for 9€/sqm, floor tiles likewise. Beige/brown. The tiler charged 1500€ per bathroom for laying.
So about 3000€ paid per bathroom.
I think you can’t do it yourself for that.
But you can tile over the old tiles. Simply apply special bonding primer on the old tiles and then glue the tiles on. Cross and line lasers are available for little money. Tile wedges, tile adhesive and grout, silicone etc. are also all available. Trial makes wise, and what you can’t do, you can learn.