You might consider using a good quality tempered glass screen protector. In addition to providing a smooth surface over which the Pencil tip will glide, the additional glass layer will protect both the original anti-reflection treatment - and the sacrificial oleophobic coating that will otherwise wear-out over time. If the screen protector itself becomes damaged, it can be easily replaced.
Plastic-film screen protectors are less able to protect an iPad screen from damage, but can provide a degree of protection from scratches and abrasion.
Zagg and OtterBox are amongst the better manufacturers of tempered glass screen protectors - with good reputation. If choosing a screen protector, it is essential that you choose one from a well known manufacturer - and that the screen protector is explicitly designed for the model of iPad.
Some users comment that a glass screen protector makes the screen very slippery when using the Apple Pencil. The alternative is a matt/textured film screen protector - but film protectors these have two principal disadvantages.
1) The textured film reduces clarity and contrast
2) Accelerated wear of the Pencil tip, necessitating frequent tip replacement.