Grounded coplanar waveguide - too wide signal wire - how to handle properly?
The easy way:

Keep your design as-is.

The wavelength at 1800 MHz is 167 mm. Your track length is 10 mm. This is less than 1/10 wavelength so you can probably get away with not worrying too much about controlled impedance on this line. Certainly the few mm of necked-down trace at the pad end of your track are not going to be a big problem.

The hard way:

If you use a thinner dielectric, the required trace width will get narrower (certainly for microstrip geometry, probably also for CPW but I haven't double-checked this). You could either switch to something like a 1 mm dielectric thickness to get a slightly narrower trace width, or make a 4-layer board to get full freedom to choose your dielectric thickness. If you go with a 4-layer board, choose the dielectric thickness (of the microstrip/CPW structure) to give you a trace width that matches the pad width.