I freeze mine, but there are people I respect that say refrigeration is enough. Protection from light and air is probably more important.

Limit how often you open containers by only diluting to final vaping strength enough to get you through a few months. That way only the small amount of diluted base gets exposed to air repeatedly; the main supply stays rarely exposed.

All nic base will darken with age - faster with light and air exposure. Pale to amber to brown. This affects neither taste nor strength.

One downside to keeping it in the refrigerator is that it is accessible to more people. Keep it in a lockable box and enclose a MSDS in the box; this is especially important with such high strength nic base. Label all bottles with content and strength. (Life is uncertain; you may not be the next person who has to deal with your supply of nic base.)

Before experimenting with flavors, I would try experimenting flavorless (to avoid distraction) with nic strength and PG/VG ratio. High-strength base is good, as it lets you reach many combinations of PG/VG at different strengths. Mix 5-10 ml of a combination and use it until it is gone (or abandoned as unusable). (I ended up at 65 mg/ml nicotine in a 68/32 PG/VG carrier.)

Back in the bad old days (1960s), I smoked a 50/50 blend of Latakia and Perique for a while. Tobacco flavors were one of the first flavors I gave up on when I began vaping nearly 10 years ago. (I eventually gave up on all flavors, and now vape just for the nicotine.)