FX 9590 overheating with H100i AIO

WHat are you using to measure temps? You should be using AMD's own utility as other's cannot measure temps properly on those gen AMD chips.
9590 is not a great processor known to be extremely hot, that said a H100i should be fine, Have you tried re applying the paste?

sell that space heater and get a zen product.

What has changed in the past few months?

Summer, perhaps?
Cooling capability, even with liquid coolers starts with ambient temperature as a base.

How well does your case supply fresh air to the radiator?
Has this been compromised in some way.
If you take the case covers off and direct a house fan at the innards, does that help.

More likely the pump is no longer operating well or is partially clogged.

When you reapplied paste, did you possibly use too much?
The purpose of paste is to fill in microscopic pits in the cooler and the cpu die.
The pits can trap air which is a very poor conductor of heat.
Paste is much better, but nowhere as good as metal to metal contact.
Usually, a small rice sized drop will spread out under heat and pressure.
It is hard to use too little.

Lastly, if your main use is for gaming, it is time to move on to a modern cpu.
Your FX-9950 has a passmark rating of 10192 when all 8 threads are at 100% busy and a single thread rating of 1725.

$200 buys you a G5400, motherboard, and 8gb of DDR4 ram. It has a passmark rating of 5255 but a single thread rating of 2200.
Good for most games, not so good for multithreaded apps.
How many threads can YOUR games effectively utilize?
Here is how to find out:
Experiment with removing one or more cores/threads. You can do this in the windows msconfig boot advanced options option.
You will need to reboot for the change to take effect. Set the number of threads to less than you have.
This will tell you how sensitive your games are to the benefits of many threads.
If you see little difference, your game does not need all the threads you have.

What you ask is not easy on that cpu. For most normal cpus you'd be looking at @100w max for Intel and 125w max for Amd. There's a multitude of relatively cheap ish coolers that'll all apply to those for testing purposes, even the original cheapo stock cooler will work to get non-oc base temps in check. However, both the FX 9 series cpus are absolute monsters when it comes to power usage and applicable temps, you are looking at 220w TDP right off the rip, and to get a more accurate look at max ability, that's peak power which clocks in at over 350w. Even the best aircoolers on the market start gagging at just over 250w, the FX 9 honestly requires a full custom loop, but can be used with a 240/280/360/420mm Aio as they'll boast 300w+ abilities.

So the only way for you to really verify it's the cooler at fault is to replace it with another of at least the 240mm h100i (that's considered basically the stock cooler) but better would be larger and higher performance, which generally have a higher price tag.

My standard advice for years has been to dump the FX 9, replace it with a FX 8350/8370 both of which are almost always overclockable to the same performance level as the FX9, yet with half the power/heat demands, where a h100i is perfectly suited, not on the lowest end of ability. However, that's a 7 year old platform and roughly 40% or better, slower than a budget Ryzen 5 1600.

So the question now becomes just how attached to that ancient monster that's fast becoming outdated and obsolete are you, as compared to a newer platform that'll last another 5-6 years. What's the better value, $100-$150 now to drag out the next 2-3 years or @$400 to last 5-6 years.

The only other thing I can see possibly that might have happened is Windows update. There was recently a major update, which has messed with a multitude of things, least of which is the power plan and it's associated voltages and current usage and instruction sets and their usage. And that all stems back to the bios. My first advice before getting into any hardware changes is make very sure you have the last bios update made for that board, and just let the bios run at factory optimized settings. If you can (doubtful with that cpu) you could try overclocking, by lowering the vcore as much as possible and still maintain stability, many have actually dropped 100-200MHz from the cpu stock settings, just to keep temps in check.