It has been a while since you posted but hopefully my story can help.

My son has SPD and is now 5. We had very similar issues getting him potty trained.

First we got him to pee in the potty by making it a bubble game and telling him it was time to make bubbles. That was a struggle but it eventually took. However it was a whole different story getting him to poop in the potty. It actually took about 5 months after he was already peeing in the potty regularly for him to poop in the potty.

The trick that worked for pooping was pretty hardcore and pretty nasty but it worked.

We spent a whole weekend not letting him wear anything below his waist. No underwear, diaper, nothing what so ever.

If he pooped and it was not on the potty he was pooping on the floor. After about 4 or 5 times on the floor our son was not happy with himself and started using the potty to go poop.

He has been fully potty trained since about 4 years old now.

None of this was easy. We had to keep trying and not give up. We had to know when to take a break and when to try again. We never forced it on him if we could tell he was getting high anxiety. But it eventually took.