Curiousity getting the better of me

Yay, my instincts were right!

For what it's worth, the monsters do get stronger, but you also get the third-strongest sword in the game at level 90, and I've never had much trouble killing dust sprites after that regardless of whether I've hit the bottom of the mines or not.
You could also consider just getting to level 115 and not going any further down until you decide you want to go to Skull Cavern. Granted, monster drops get a bit better once you get to the bottom, most notably, after getting floor 120 all monsters have a 0.05% chance of dropping a prismatic shard.

You could pick Prospector and then clear your farm, I guess, but since the chance of coal only goes from 3.5% to 7%, I wouldn't expect that you'd get much more coal than you would from a day killing dust sprites. And it costs 10,000g if you want to switch perks after that - you could just buy 40 coal with that money.
On the other hand, Miner (the level 5 perk for more ore) is very useful, so if you're not selling any metal bars (and therefore not taking Blacksmith) there's an argument to be made that Miner > Prospector is the better set of perks just for Miner.

 

i always take long with the mines, and kill as many dust sprites as possible per day, and even with timespeed mod, i need to use the burglar's ring for enough
i may play differently, where i want a lot of money but also a lot of playing around, different characters with different farms, adn since i started using bee houses with the best flowers for them, i'll place a lot of them, needing 8 coal each

killing dust sprites gives the burglar's ring, then killing them is way more coal, but i gotta do it quite a lot of times, just to get what i want/need
i put the mod on 20 ticks, meaning every 20 reallife seconds are 10 minutes in game, and still takes a LONG time to get ahead of my crafting and smelting
and if i need a bunch of coal quickly, i'll buy them in year 1, as in year 2 prices go up

Miner > Prospector? if you get more ore, you get more bars, and most stuff needs copper bars, so selling iron bars from ores you get in the ice areas, where you kill the sprites, would be more effective, i think?

and yes, diamonds and prismatic shards drop from all monsters after level 120, and with the burglar's ring, the chance is higher, so more money

but that's what i'd do.. i just never get Miner, so i only speculate your case
also, more gem stones appear if you enter the mines with a + Mining buff (and lucky day, if possible) on you so maybe using Miner's Treat before entering would be moneymaking, a bit more than otherwise, i guess

ps. Burglar's Ring doesn't stack, so using 2 won't help more than using 1

 

Aaaand two days after I said this, Prospector got fixed so it worked everywhere.

 

Ooooh, I missed that from the changelog. That is great! (I don't have it yet (not level 10), but it was always the idea to pick it after Mining).

Edit: I also noticed quite some talk about how to get rid of the slingshots by using chests that NPCs walk over and destroy. That option is now gone. The contents of the chest will remain. But.... we can sell the darn things!