IMO, I don't see much you could upgrade because your system is very good for 1080p 60hz gaming. Whatever upgrades you make should be based on whatever kind of monitor you are going to use. Perhaps saving the $300 then waiting until next spring would be more beneficial because there should be new and better hardware out by then.
The only thing you didn't mention was your monitor, which is one of the most important aspects to consider when you're making upgrades.
For example, if you upgraded to a 1080p 144hz monitor then upgrading the RAM to a DDR4-3200 cl14 kit such as the G.Skill Flare series along with giving the CPU a modest overclock would be beneficial. You would need to overclock in a manner that matches what Precision Boost 2 does on an x470 chipset motherboard, which would involve having a couple cores reaching 4.3ghz while increasing the base speed a little, if possible.
If you upgraded the monitor to a 60hz 2160p or 1440p then you wouldn't need to worry about overclocking because the Ryzen 5 2600 is more than enough. You need more CPU power when you're trying to push your framerates beyond 80-90fps.
As a general rule of thumb, you choose your GPU for the settings you want and you choose your CPU for the FPS you want.
Having the CPU's Warranty get voided by overclocking would only happen if you did something blatant such as overvolting the CPU. When a CPU is overclocked within specifications the chip degrades a little bit faster than normal. A CPU at stock speeds can last well beyond 20 years. If you're going to overclock then you need to read a few guides and figure out what the voltage limits are and how to make sure the CPU is stable. You can't just bump up the multiplier and then expect magic to happen and I've actually seen people fry hardware this way.
Crossfire????, you might be able to *** a 2nd card for Crossfire but there aren't that many games that do a good job of supporting Crossfire or SLI. I'm not quite certain how you would run Crossfire since you already have an NVMe drive that's occupying 4x PCI-E lanes. According to what I've read so far about your Pro4 board, it wont work because of the NVMe drive. Also, you would need a beefy power supply to run Crossfire, in the 750w range. Crossfire for you would probably lead to a lot of headaches.
A GTX 1080ti would give you better performance than trying to Crossfire 2 of those cards. A single GTX 1080 would only be weaker than an RX 580 crossfire setup in a certain handful of games. The GTX 1070 would be an upgrade but I don't think it's a good enough upgrade to justify. If you're getting 45fps with the RX 580 then typically the GTX 1070 would bring you close to 60fps, since the GTX 1070 is 33% faster than the RX 580 in gaming.
On the upside, mining isn't quite dead yet. Right now miners are buying RX 580, 580, 570 and 470 cards people who are giving up on mining. A used 4gb RX 580 is worth about $150. You could sell that card to help pay for a GTX 1080. You could always buy an EVGA card then use the Step Up program to upgrade to an RTX 20 series card if you do it within the 90 day window. The only downside is you have 14 days to register your card with EVGA in order to take advantage of the step up program. 30-days if you're willing to buy an extended warranty. The RTX cards aren't cheap, as the RTX 2080 is $750 at the cheapest, and it looks like Nvidia is going to push back the release of the RTX 2070 for as long as they possibly can.