I've just gone the other way - from a "normal" 12mm to 6mm drop. I didn't anticipate any difference (what can 6mm do ?) but was quite surprised.
I noticed that there was a slight adjustment period - nothing huge, but my back and calves ached more than usual for a couple of weeks.
The main difference though was that I run more smoothly with the lower drop - less of a heel impact/strike and I found I naturally tended to run a little faster. I don't know if the reverse would be true - more impact and slower if I went back to my old shoes, but going to a lower drop has certainly been positive for me.
If you have issues, there are lots of trail shoes with a lower heel. Inov-8 for example do a range of different heights.
I don't think there are any hard rules - shoes will last different times for different people depending on weight, running style etc.
Like you, I change mine every 300 miles whether they are obviously worn out or not and for the same reasons as you. I'd rather lose the £10-20 worth of running that may be still in them than get injured.
Flob - good luck ! You're about to find out the downside of Hoka (at least in my experience). They are VERY difficult to find, particularly if you are a common size. I've only ever been able to order online.
I did hear they've been bought recently by a much bigger company, so maybe things will improve soon.
My recommendation if you order online is go with your American size, not British. Typically you need a half size bigger than your British one.