I'll give it a shot.
firstly, I'd like to address the 15% duty that you pay for importing books into the country by stating that you're not alone. In fact, far from it as most countries require full declaration of imported items for duty. 15% may seem steep, but its not the highest duty amount placed on imported goods, especially in comparison to some European countries.
Secondly, ASM 300 has seen a sharp increase in value recently for several reasons: Venom movie, covid stimulus funds, market catch up (it plateaued for quite a while previous to this run up), and just a general collective awakening of the importance of this book. As prices continue to rise (you will see more strong results this week), more copies will become available. We've actually started seeing this on ebay and elsewhere. For example, look at how many copies are available currently vs two weeks ago and you'll see the difference in availability for yourself.
Eventually, and I'm not sure when, the book will plateau for a while, and once people realize/are aware and comfortable with the new pricing 'norm', the book will rise again. Rinse, and repeat. We've already seen evidence of this on Hulk 181, which continues to reach new highs vs ASM 129 which is currently experiencing a plateau in its highest grades. Other factors may also come into play regarding demand for the book, such as what are Disney's plans for the character? I imagine demand would spike further once they make a TV/Streaming/Movie announcement?
Ultimately, the decision comes down to your current financial decision...do you need the money now or can you ride this current upwards trend until it plateaus?