I swear by “Lactulose” an oral solution you take after a meal. It’s very sweet and it draws water into the bowel. I had constipation bad for 6 or 7 years (wasn’t diagnosed with PD at the time) and after trying various Laxatives with little success the Doc prescribed that. I tale two 20ml doses, one after lunch, and one after evening meal, and I’m fairly well back to normal. I ran out over the recent B/Holiday Monday, and immediately started heading back to straining. Got some Tues, and by Wed night back to normal.
You can buy it over the counter if Doc won’t give a script (£<5 ish for 500ml bottle, I get 2 a month)
As an aside: pre-diagnosis I had constipation and loss of smell, the latter for 10 years+. Once handwriting became small and tremor started I already knew before the specialist confirmed it.