There’s no guarantee that PLEX will always go up against ISK, and hasn’t done so much as of late, but at least it will always be worth something as you can trade it for game time.
What things are worth can change on a dime depending what CCP does to the game. T2 BPOs could be nerfed or inactivated tomorrow. Rorquals could be nerfed or buffed changing what minerals are worth. Moon goo was recently changed and could be changed again. At least you know that 500 PLEX will always get you one months game time.
To preserve value, I would go 1/3:1/3:1/3 with a third in a basket of core materials - minerals, PI material, T2 moon materials, salvage, etc., a third PLEX and hold the last third in ISK. Or even easier, skip that materials and just go 50:50 ISK and PLEX.
If you want to actually invest in something, you would need to identify something that will become rarer (hard to do as CCP can and does re-release things) or is going to appreciate in value due to upcoming changes to the game or recent ones that haven’t been fully felt. I think this is hard to do under the best circumstances, but even harder if you don’t know when you might come back. I think I would just diversify my assets instead of trying to pick something that needs to be sold at a particular time to realize a profit.