Selecting true crystal settings for 328P [7.3728 MHz]
The external capacitor selection is not so accurate, and the 18pF value for pin capacitance is from the table for using it with 32kHz tuning fork crystals, not for higher frequency crystals.

You have a crystal with CL=18pF. AVR datasheet says to use identical capacitor values between 12pF and 22pF. If you put 22pF capacitors there, it creates 11pF of load, if you assume the pin and stray capacitance to be 5-10pF in total, you end up with 16pF to 21pF of load which is very close to the required 18pF. It will certainly just simply work. For any more accurate capacitor values, and to include larger variations, you would have to find a tool to measure the frequency or estimate the PCB stray capacitance. But as you see you can't really put a crystal with higher CL rating there, so if you have higher stray and pin capacitance, perhaps using 18pF capacitors and crystal with CL=16pF would be better, but overkill as a starting point.

You can also keep using the internal oscillator if you use USART U2X bit and calibrate the internal oscillator down to 7.37MHz.