Do not understand how to use Portal at >300 power

I hit 300 power shortly before Renegades came out. I noticed that trying to do Portal activities required MANY more negative modifiers to give me a A rating. Even a B rating was very challenging to get. But I figured there would be a power reset when Renegades launched so I took 2 weeks off and didn't worry about it.

Well, there was no Power Reset, and Portal Activities are basically unplayable. Quick play is unusable. I already read about that. And that's not a big deal for Solos. Solos I can generally get to A.

But Fireteam and other group activities I need to customize (which means no matchmaking) to get above a C. I obviously can't use Quick Play here either, but even the matchmaking for the selected games, aka no customization, doesn't get above a C. And I'm just barely over 300 (like 306), so the idea that I should be kicking these activities to the 400 level seems insane.

So what am I supposed to do? How am I supposed to get match made group content with a reasonable scoring ceiling? Is that just not something Destiny 2 supports? Do you need premade for every group activity?

I'm very confused and feeling like Destiny 2 isn't a very solo friendly game right now, which is making me want to quit. Any help would be appreciated.