I'm all for upgrading cheap big box bikes but as soon as you add dual suspension into the mix I just feel it's wasted time and money because the suspension is next to useless, has no benefit to riding, wastes a huge amount of rider power and throws up problems and can end up with the bike being scrapped fairly quickly.

Many such bikes have terrible freewheel wobble because the threads on the rear hub and possible even inside the freewheel aren't quite parallel so there is wobble and it doesn't matter if you put a Deore rear derailleur on there it cannot compensate for the freewheel moving in and out and this can make adjusting difficult and mean the tolerances of adjustment have to be so perfect to get good shifting.

If I had that bike and had to use it I would spend serious time adjusting everything to the best possible state and look for a cheap freehub based rear wheel that I could put a 7 speed cassette on. I might also look out for a cheap ok front suspension forks like Suntour XCT or even go rigid for the front forks and end up with a weird soft-tail bike. While I don't like these cheap dual suspension bikes I often find the rear suspension is better than the front which is often too weak to be safe and bottoms out far too easily. I remember adjusting the rear suspension on such a bike to the highest tension level and it could be adjusted to provide a fairly ok ride only really moving with an impact to the rear, it was a heavy bike but did work as intended and seemed strong. Of course there is massive variation in such bikes I'm sure just my experience.

The cheap rigid framed bikes from walmart or other sources can be upgraded to nice bikes with the right parts and ride very well. They might be a little heavy because of their un-butted simple frames but still make very nice bikes in my opinion. Good workhorse bikes especially with their abusable steel frames.