The main problem I have seen with trying to upgrade these lower-end suspended frames is that the shocks they use are really short.
You can swap in a low-end Rock Shox fork easily enough, but it's the bike's FRAME shock that is the problem.
This level of bikes adapt the "obsolete" parts of more-expensive brands several years ago, so they lag in tech, and bicycle shocks are getting longer over time.
The longest frame shocks you will find on these cheap bike frames are around 150mm long, while the shortest name-brand shocks you can buy from a bike shop (Rock Shox, Manitou, RST) are up around 175mm now.
If you can weld then you may be able to fix that, but it doesn't stop bicycle shocks from getting longer over time (the longest ones now are 250mm I think,,,).
If you weren't real serious about the performance, you could just switch it to using minibike shocks. They have been 12 inches eye-to-eye for about forty years now, and there is an oil-damped one that only costs like $40.