Looks like Sefer uses a similar strategy to the one I use. I upgrade all my mines to level 7 up to about Neodymium, and then 2-5 all the way to the end. I end up having almost as much ₩ at the end from ore sales as I do from selling items. I just finished Africa, and the deference was just under 700 billion vs just under 900 billion.

@Sefer, do you find level 4 supplies enough for all your recipes for techs, or are you past your first ascension?
Also, what are your AP upgrades in?

I get all early mines to at least level 4, and push them higher as needed to support whatever I'm crafting. It's just a baseline I aim for because I know I'll need it eventually and level 4 is the point that the penalty in mine math stop applying. (Don't remember the formula off the top of my head, but I've seen it multiple times on the forum). Several mines will get to level 7 or higher, if they're particularly cheap (sometimes I max out the very first mine if I need a lot of copper ore for a later bar- if it costs less than a second or two of my income I just go for it) or particularly necessary (I push a lot of silicon and aluminum since I like to have 4 or more smelters of each going at all times).

I was just starting the last level I'd need to ascend the third time when the new update came out, and I've abandoned ascending for now to work on hardened levels. I've just unlocked the 4th advancement tier. I've got AP all over the place, but the ones relevant to the discussion are probably maxed Increased Ore Sell Values, Discounted Mine Upgrades, Increased Cache Money, Increase Smelters Speed, Increased Cache Resources, Increased Money from Bonuses, and Mercenary Discount plus Additional Mercenaries at 72.5%, Increase Crafters Speed at 35%, and Increased Item Sell Values at 75%. I'll add more to those last three eventually but my economy and merc boosts are on pause while I work on Better Hospitals and Increased Cache Armies now that those are available to me. It's been a long time since I ran out of mercs (I actually bought all of the mercs from Hardened Peloponnesian War earlier today, but it was way more than I needed to finish the map) so it's been a long time since I've felt any pressure to upgrade that.

I started a new forum thread dealing with crafting smelting strategy
https://www.warzone.com/Forum/568266-new-warzone-5140-smeltingcrafting-guide#Reply

I know, we agreed on the fact that unlocking higher phases isn't the first priority you should have, but I just did the math: With the new hardened levels (and assuming you start them even if you haven't reached their recommended level of advancements) you can in theory reach phase 4 with a single play through. Though, I really wouldn't recommend this, as IAP would be (except for 300 additional AP) the only thing you would invest in in phase 2.