I find much of your advise to be very timely, but with due respect, a bit late for anyone who may have built or added to a collection in the past 24 to 36 months. If they were not being selective, buying used or agressively, and buying what they really, they are a bit trapped now.
My 2 cents:
Have a criteria for all purchases and follow them. I started this 2 years ago as a way of making sure what I bought was really what I wanted. I have 5 criteria including size, functions (date), style, movement, and scarcity.
The last of these, scarcity, is why of my 7 piece watch collection, all are numbered, limited, and/or hard to find.
My MIH is the most distributed watch I own with about 700 (and growing) others in the market. Even with so many others, and the ability to order new still, it has retained much, if not all, it's value. Among the others (a chopard luc, delaloye, UN marine acqua, UN michelagelo), all are from editions of 100 of less, or from limited production makers (speake-marin, sarpanova)
Not all criteria are met each time, but 4 of 5 are at least, and several are limited edition versions of more mass produced watches but the law of supply and demand remain.
The "turn" in the watch business is famous. The collectors market here is fill of "worn twice, like new", "making room for new", etc. I am amazed sometimes by the short-term view that some bring to their collections.
Anyway, my point is that lessons learned in challenging time need to be honored in good times as well. When you start to see these lessons being forgotten, or hearing "it is different this time" you know there is a bubble.
Have a purpose for your collection, and don't make exceptions. Buy for the long term, but like you will have to sell tomorrow so "hedge" you purchase at the time you buy by getting a price that removes depreciation, etc.