Kyle T. Webster Brushes are pixelating in Photoshop CC

Using Photoshop CC 20.0.5 on iMac (macOS Mojave Version 10.14.5)

I installed the Megapack this morning and have had several other of Kyle's brush sets installed, and they've all worked fine until now. For some reason, any brush I use, rather with a Wacom or mouse, appears horribly pixelated. I was first creating a character for Character Animator (1170px x 1270px @ 72dpi):

So I thought maybe the document size needed to be changed, so I made a new doc 3000px x 3000px @ 300dpi, just to check the brushes:

It still looks jagged. Also, the preview of the brushes in the Brush preset picker look pixelated, too:

I have restarted Photoshop, and restarted my machine. Any help would be very much appreciated, as I need to get my project going and done!