What can/would you upgrade a SRAM GXP BB to?

If you're trying to find an improvement over the bottom bracket, you probably won't find such an improvement.

The bottom bracket shell has a fundamental flaw. It does not allow a durable spindle and durable bearings inside the shell at the same time. Durable bearings require big enough balls. Durable spindle requires big enough diameter.

Originally, the square taper system used a small diameter spindle. The spindle failed, often times catastrophically, hurting the rider.

Then Shimano came up with Octalink. It used a large diameter spindle (thus the spindle did not fail), but required small bearing balls because there was not enough room for big bearing balls. Not only that, but the crank - bottom bracket spindle attachment was fundamentally flawed and the crank bolts loosened in use even if originally tight. So the bearings failed quickly and the cranks self-loosened.

Later, Shimano came up with Hollowtech II. It moved the bearings outboard out of the limited size bottom bracket shell. The spindle is attached to the crank and has a large diameter. Because the bearings are outboard, the bearing balls can also be large at the same time with a large-diameter spindle. It more or less eliminated failures.

The SRAM GXP bottom bracket seems to be the SRAM equivalent to Shimano Hollowtech II. Thus, at least in theory it should be durable. There is no "upgrade", i.e. no identifiable design flaw that would be corrected in another product.

Thus, I would select a similar bottom bracket. I also would be very careful in its installation instructions, i.e. set the preload correctly. A misadjusted preload can of course cause the bearings to fail sooner than expected. Use torque wrench during installation!

The SRAM Apex crank requires a GXP design bottom bracket., i.e. 24mm drive side and 22mm non drive side. That's not something you can change.

The GXP design is generally reckoned to be inferior to Shimano's Hollowtech II design because the non-drive side bearing floats and is not pre-loaded, unlike Hollowtech II where both bearings are pre-loaded. Source: Hambini YouTube Channel Caution! Lots of swearing and cussing.

However, you could look for a product with higher quality bearings than SRAM offers. There are several 3rd party manufacturers that make GXP compatible bottom brackets. Wheels Manufacturing for example.

You should also check the alignment of the threads in you bottom bracket shell. Angular misalignment of the bearings accelerates wear. One way check (at least for gross misalignment) is to remove the crank, then re-install. If the non-drive side end of the axle is not lining up exactly with the non-drive side bearing as the axle is pushed through the shell you have misalignment.