Lots of great suggestions for better places for these various bus companies to stop but I think the more important conversation is why aren't they using them?
Flixbus for example used to stop on George St. in the heart of the Market (using designated coach/tour bus on-street parking). This location had many attractions/amenities/accommodations within walking distance, including Rideau station.
They then decided to move to a torn-up Commissioner St. location with zero on-site signage and a very vague pin-marker on a map included in their confirmation emails. I recall seeing Reddit posts from confused riders having trouble finding the stop.
Then they moved to the most remote part of the massive parking lot of Pinecrest Mall. Far away from the OC Transpo bus stops on Iris St.
For Flixbus routes that take Hwy 7 and also stop in Kanata, they use an
effing gas station (the CT gas station at the corner of Terry Fox / Didsbury). I once saw a confused looking woman with a suitcase standing at that intersection. I pulled over to help her, and when she told me she was looking for the Flixbus stop, I told her I was 99% sure she should be heading to Terry Fox station which has (or used to have) one of its stops reserved for inter-city buses. She then pulled up her confirmation email, which, to my surprise, showed a pin marker loosely dropped on the location she was standing. While we were trying to figure things out, another lady got out of an Uber with a suitcase leading us to believe that was indeed the correct location. I was utterly shocked.
So clearly there is a reason these companies are using the worst pick-up / drop-off locations they can think of. My guess it has something to do with avoiding whatever fees they're charged for using more connected locations. As I mentioned previously, even in Toronto a lot of these companies don't use the Union Bus Terminal which was purpose built for regional / inter-city bus travel.