If you did want them to professionally remove the microphone/sensors maybe it’s worth it (still very pricey though), but if you’re simply going to install GrapheneOS then you can certainly DIY it.
Having the microphone removed isn’t completely valueless, and you’d still be able to make calls by simply plugging in a USB-C headset/earbuds as needed, which acts as a physical kill switch of course. Most people will just rely on the software microphone/sensor toggles in the Android control panel and call it good enough though. The risk is next to 0 and thus probably not worth an extra $600 + inconvenience, unless money is no object at all.