There is no "one size fits all" upgrade path, but there are different strategies that might make sense to follow:
Upgrade as You Break
This is probably the most conservative approach to upgrades and most likely what I would recommend to newcomers to the hobby: You replace parts with better ones as they break. This strategy has a natural logic to it - the parts that fail are revealing themselves as the weakest links in your setup.
For example: The driveshaft is a likely part to break, so you replace it with a metal upgrade. Now that the driveshaft is stronger, next time something breaks, it'll be a different part - maybe the transmission gears, the output shafts, a motor mount, or even the axle internals. You're essentially moving the weakest link up the chain of components.
Here is where it gets interesting though: You might be moving that weakest link to a potentially more expensive part to replace. A plastic driveshaft might cost $5, but if upgrading it means your $30 transmission starts taking the abuse instead, you need to weigh that carefully. So it's not always the best strategy to automatically replace every broken part with an upgrade - sometimes a stock replacement makes more financial sense.
Better Performance
With this approach, you upgrade parts specifically chosen to improve performance, regardless of whether anything is broken. This will vary from model to model and person to person based on what you want your rig to do.
For me personally, I like to start with heavier brass wheels or brass portal covers. The added weight lowers the center of gravity and improves crawling performance dramatically. But this almost always requires a better servo as well, since those heavier wheels - especially with grippier tires - create much more resistance for the steering to overcome. You can see how one upgrade cascades into another.
Other popular performance upgrades include: better shocks for improved articulation, stickier tires for more traction, a brushless motor setup for more power, or metal links and steering components for precision and durability.
Fixed Budget
You set yourself a fixed budget for your SCX24 and its initial round of upgrades, then choose the parts that give you the best bang for your buck. This forces you to prioritize and really think about what will make the biggest difference for how you drive.
Maybe that is $150, maybe it is $250 - whatever you are comfortable with. Then you research and select upgrades that work well together and give you the most noticeable improvements for that money. This approach prevents the endless "just one more part" spiral that can happen in this hobby.
Planned Build
This is the opposite of "upgrade as you break" – you have a vision for what you want your rig to become and you systematically work toward that goal. Maybe you want a super lightweight trail runner, or a comp-style crawler with maximum flex, or a scale-looking build with all the details.
You might create a list of all the parts you want and prioritize them, buying a few each month until the build is complete. This prevents incompatible parts or redundant purchases, but requires more research up front.