In the most simple terms, yes, it’s fine and normal to run everything on SSDs now. As alms says, the only advantage to HDDs is cost per storage.
What I’m not completely sure alms is getting across is that basically there are two kinds of ways data is accessed (simplifying hugely) - either sequentially, where stuff that’s “next to each other” on the disc is accessed, or randomly, where that doesn’t happen. HDDs are fine at sequential, but because they have to physically spin discs, terrible at random access. SSDs are better at both, but where it matters is the random-style access, because the difference can be absolutely gigantic.
Sequential access is basically, for home user purposes, recording/playing back stuff (films, media, etc.) - there’s a bit more to it of course but that’s basically it. That’s what HDDs are fine for.
Whereas virtually everything where you think of a “loading time” or “loading screen” involves random-style access. Even an SSD that isn’t much better than an HDD at sequential access will be drastically better at this. We’re talking several times faster. NVme, on paper, is several times faster still, but in practice, because of other limitations to throughput, is usually noticeably faster but not like, the several times the on-paper number would indicate.
A good example of particularly random access would be loading into a city in World of Warcraft. With an HDD, this could take 30-60 seconds (or longer!), with a both a loading screen, and then huge amounts of FPS drop, and then textures and models only gradually getting rendered in as the game works out what it needs over another 30+ seconds. With even a crummy 2016 SATA SSD the loading time dropped to about 5-15 seconds (with the new issue being the server not responding quickly enough rather than the data taking a long time to load), and when it loaded, there was between 0 and 3 seconds of “gradual rendering”. This was with an aging mobo, memory, processor etc., and is not in any way an exaggeration.
Basically any game where you’re frequently loading a bunch of stuff is going to see big gains - and not just in the initial load. Open-world games will benefit significantly, especially ones where you can travel at speed, or fast-travel, and you’ll see a lot less texture pop-in and so on.
If you can afford it, I’d suggest an M2/NVme drive with your system and also put any really load-heavy games on it (unfortunately these also often tend to be whoppers in GB size, which is irritating), but honestly if you just get one big enough for the system and buy normal SATA SSDs for the rest you’ll probably be extremely happy. I wouldn’t bother with HDDs unless you intend to record or store a lot of stuff.