Some numbers :
a) Your winner spec of .02s/1800 = 11.11 parts per million (!)

b) 90*16124 = 1451160 ( appx 21 bits )

That figure in a) is already pushing Crystal specs, and you are into TCXO (or better) regions. Look for something like FOX924B or better

1-3ppm is affordable (sub $10), and fractions of ppm, are available.
eg Digikey shows 100ppb for $23.70


- but I'd check just how good the master clock really is, and getting a course distance to 11ppm, is also a serious challenge.

Traction creep, tyre wear, tyre pressure, and even lines taken thru bends, will all change your expected 1451160 pulses.

Can you do any way-point checks ?
How straight and predictable is the final part of the course ?

b) means you will need 32 bit maths, but you are best to run that at your ~10 times a second, not the 806+ pps pulse rate.
That means you would capture a Hardware counter, every ~100ms, and add that to your 32 bit running total. (you add an average of 80.6 each time)

The counter needs to be configured for External Clock, and Hardware Capture, and in the SiLabs parts, that means use the PCA, Clock via ECI, and HW capture via CEXn, [Capture triggered by transition on CEXn], and the 100ms loop, would toggle the CEXn you chose, to perform the capture.

There is no capture-and-clear in HW, so each capture needs to subtract the previous one, to give the new pulses in that 100ms window.