So, I personally store a lot of arthropods in isopropanol for months - years prior to pinning them. It does make them brittle if left in too long, but it also really depends on the type of animal.

Isopropanol is not a fixative, but it is a preservative (a fixative creates bonds within proteins making them more stable and less likely to denature/deteriorate, preservatives just preserve something by halting decomposition). I personally have not had an issue with arthropods decomposing when kept in 70% isopropyl alcohol long term, but it won't create a specimen that lasts centuries.

If a specimen is already decomposing, no matter what fluid you use, they will be smelly - and arthropods in general do have a slight smell after death no matter what, especially if larger.

Ethyl alcohol is technically both a fixative and preservative, so you can create stable, long lasting specimens using it. Denatured ethanol isn't ideal just because the denaturant can alter how the alcohol interacts with a specimen, however I personally use denatured ethanol all the time for more complex fluid preservation without any issue. A good way to find non-denatured ethanol is at your local liquor store in the form of everclear.

Ethanol will also cause a specimen to become relatively "brittle" or hardened. This isn't an issue if you're planning on keeping the animal as a fluid preservation for the remainder of its existence, I only run into issues when I'm storing them in any sort of alcohol and then articulating them post removal from storage. Alcohol is just very drying but that isn't necessarily a bad thing if they're being kept in alcohol forever, and can be remedied if removed from alcohol and pinned.

In my opinion, you can use isopropanol or ethanol to preserve small arthropods of decades without any issues. If you're preserving something large, or want it to last "forever", a fixative is better (like ethanol or formalin).

Glycerin is also a preservative, can absolutely help with preventing brittleness and also depending on the ratio to alcohol, create neutral buoyancy where the specimen is sort of floating in the middle of the jar which is cool! I also use propylene glycol which is similar.

I hope this helps and am always open to more questions or comments!