Had to reset my bios last night on G436. Now performance is lacking

Welcome to the forum, @OhGodIThinkIBrokeIT . Clever name by the way, given the context! Worry not, I should be able to walk your G436 back to the way it's configured out of the box in terms of BIOS settings. There isn't much that we do on the production line once the BIOS is flashed on these units aside from manually changing the RGB settings to a "wave" style and loading the XMP profile, which it sounds like you may have already done. Just to confirm, when you say you changed memory to 3200mhz, you did load the XMP, correct? Simply adjusting the frequency without loading the profile may load some atrocious primary/tertiary timings and cause this lapse in performance, so it might be best to start there.

Navigate to your BIOS again (F2/DEL at boot) and press F7 for the advanced menu. Enter the OC subsection and scroll down to the Intel Extreme Memory Profile (XMP) settings and make sure this is set to enable. It may give you options like "XMP 1, XMP 2", etc. Load XMP 1 if you have a choice, otherwise, the standard profile should be the only option. From there, press F10 to save & exit. The rest of our defaults should already be loaded after your original clearing of CMOS, we specify what these default values are to the board vendor, so this should restore it back to that configuration. The CPU frequency of 4.8ghz sounds normal based on your temperatures (standard all-core turbo is 4.7ghz for the 10700K, but can go higher based on power/thermal overhead).

Once you confirm XMP is loaded, give your games another shot and see if your original performance has been restored. If not, we can dive further into this issue and figure out exactly what is at fault. Best of luck!

These are great questions.

You're correct, the Z490 Plus used in the G436 is actually known as a "System Integrator" board, meaning it is only sold and used by system integrators for mass production and as such, does not receive OEM support by MSI but rather the manufacturer of the system it is used in (in this case, PowerSpec/Micro Center). I wouldn't consider the board "proprietary" per se, but I can understand the hesitation to use such a board given the sparse availability of BIOS updates and end-user support. That said, we do take care of our systems over time and should you run into anything with your system, we'd be happy to address it as best we can. As for your questions in particular:

1. You could, as MSI's trace topology for their non-MEG boards are fairly similar across the board, however I wouldn't put too much weight into QVL's. Memory not being on a QVL doesn't automatically mean it won't work, nor will memory that is listed on a motherboard QVL be automatically guaranteed to work. QVL's simply mean that the traces on a board are verified to support a rated speed and that they tested memory with a specific memory IC (Samsung, Hynix, Micron, etc) in a specific configuration (2x8, 4x8, 2x16, 4x16, etc) and it worked. You still have variables such as processor memory controller quality that can prevent a given memory speed from working, or introduce memory instability despite buying a high-speed memory kit and a board rated to support said memory. I have personally tested this board with a 2x16GB 3600mhz C16 kit without issues, so I can say it's capable of handling that speed, though again, your processor memory controller quality may vary. If I had to make a safe recommendation, I would aim for something like this: https://www.microcenter.com/product/611889/gskill-trident-z-rgb-32gb-(2-x-16gb)-ddr4-3200-pc4-25600-cl16-dual-channel-desktop-memory-kit-f4-3200c16d-32gtzr---black. You can also opt for a cheaper 16GB kit, but that would be a compromise on capacity, and might be a step backwards in performance for the sake of aesthetics. Make sure you clear your CMOS when installing new DIMM's and reload the XMP as the old profile may attempt to load, causing instability or POST issues.
2. I have never actually tried to swap the front panels between the ATX 205 and 205 Mesh, I would need to check and see if their internal tooling on the front panel clips are identical. If they are, I wouldn't see why you couldn't simply swap the panels around. As to where to procure a front panel by themselves, that I have no idea of. I can say that if you are not feeling the ATX 205 chassis used in the G436, you are absolutely able to swap the internal components into another chassis without voiding the warranty (as long as nothing is damaged in the process). Just make sure you hold on to the original chassis as it may be needed to process any future warranty claims as all serial information is located on the original chassis.

Also, excellent choice on SeaSonic for a PSU brand, I am most definitely a fan of their units, same goes for SuperFlower (EVGA's original OEM). Since you mentioned you have the Zotac card in your G436 build, it's only fair that I warn you that clipping those 6+2 pin PCIe power connectors is a real pain. The cards heatsink has a recess that those connectors plug into and keeping the 2-pin connected to the 6 pin can be a bit of a hassle. Hopefully the unit you buy has solid clasps on the 6+2 piece, otherwise you may need to improvise with a piece of tape or zip tie to hold the two pieces together, plug them in, then remove the tape/tie.

So there is a difference with going with a 4x8 vs going 2x16. In a 4x8GB configuration, you'll be using 4 "single rank" 8GB DIMM's. In a 2x16GB configuration, you'll be using 2 "dual rank" 16GB DIMM's. Without overcomplicating the explanation, dual rank DIMM's grant you access to a feature known as "Rank Interleaving", which allows you to write to one memory rank while still reading from another. This gives you a general reduction in overall memory latency while giving you a boost in copy memory bandwidth, so there is definitely a benefit to running in this setup over 4x8. You also get the added benefit of increased capacity in the future if you wanted to go with a 4x16GB setup later on.

That said, my complicated explanation only really amounts to a 5% difference in real-world performance in the best case scenarios, so if you are sweating the decision, it's not that big of a deal in the grand scheme of things. If a 4x8GB configuration is more easily accessible and better suits your budget, go with that, especially if it meets your aesthetic needs. I am a bit of a memory nut and enjoy memory overclocking in general, so I tend to harp on every little bit of memory performance when it's not really necessary.