The original Surface pen works well with the SP6. There are a number of 3rd party pens that work just as well with same 4096 resolution, programmable buttons, replaceable tips, magnetic storage. The advantage of some of the other brands is they are rechargeable. My original pen died after a few years and I replaced it with this one:

RENAISSER Raphael 530 Stylus Pen... https://www.amazon.com/dp/B09LQF1VLS?ref=ppx_pop_mob_ap_share

It's been great, feels and draws like the MS pen - no regrets.