Well, going from an HDD to an SSD based drive is a HUGE step up and you will immediately notice the difference. SSDs are fairly cheap now and are for the most part extremely reliable, so I'd say it's a pretty good time to ditch mechanical HDDs.
Regarding Raid 0, most people will opt for that option when they go with standard SATA based SSDs. It can provide a nice boost in performance in some cases but just like any Raid 0 array (SSD or HDD) you will have to deal with it's potential downsides.
M.2 SSDs come in two flavors... SATA based M.2 and NVMe based M.2. Basically, M.2 SSDs that are SATA based are SSDs that are pretty much the same as standard SATA SSDs, they simply have an M.2 interface. On the other hand, NVMe M.2 SSDs share the same interface (M.2) but are not limited by the SATA protocol since they use NVMe and have the potential to be much faster (which they typically are). The main downside with NVMe SSDs though is that they are fairly expensive. I'd even say that they can get into the realm of diminishing returns in terms of "gaming" performance since even if they have 5x the sequential read speed of a standard SSD, your games won't necessarily load 5x faster type of thing.
IMO the best bang for the buck in terms of performance/capacity is by far going with a standard SSD. In your particular case, going from a mechanical drive to an SSD for your main drive will definitely provide a huge improvement, even if you decide to go with a single SSD instead of going with a Raid array.