In the fall, leaves and acorns.
In the spring, oak pollen thingys.
One thing that helps is having larger gutters. That is, 6 inch K style vs 5 inch. With accompanying larger dropouts and downspouts (4x5 instead of 3x4). The larger dropouts decrease the "bridging" over the dropout.
The solid cover type gutter guards will be most effective against acorns. They will roll right off. Also generally good against larger leaves, like the oak leaves. As you found out, pine needles go right into that 3/8 inch opening and clog.
Because they are professionally installed, and most are permanently attached to the gutter and roof, they are hard to get into if you need to clean your gutters. There are varying opinions on these out there, particularly on the quality of the local installers. That seems to be a big variable.
Most of those mesh covers are pretty flat once installed, so debris tends to accumulate on top of them. However, big chunky debris (like acorns) tends to blow off eventually. Big leaves can blow off, or can get wet and matted on top, keeping water from getting into the gutter. The oak catkins (pollen thingys) tend to cling together because they have lots of soft spikey parts, sorta like velcro, that help them collect into mats.
The mesh covers are DIY installation, and will cut down on the debris in the gutter. They need to have debris on top cleaned off periodically. They are also a lot easier to remove and clean under if they don't keep everything out.