A few changes I’d make next time:

• Do 1 or 2 rows of garter before starting short rows to mask the “steps” and smooth out the line between the border and the body.
• But really, I wouldn’t do garter stitch for the body. I’d repeat the last two rows of the border with a p1 instead of the p3tog. Then the vertical lines of the border would just continue into the body, with a stockinette background.
• increase the short rows by 2 instead of 4 for a deeper center. This one is very narrow. It’s okay, but I think I’d prefer it a bit wider. Perhaps start by increasing the short rows by 2 and move to 4 as the turning point moves to the edges.