There’s a few public mines scattered around the portal network hubs on T5/T6 planets for gems, and @bucfanpaka has a whole network of sov planets with public mines which are really good with all the gem types. Metal ores, you usually get them as a side product from mining gems. Since they don’t really have a set area they spawn in as long as you’re deep enough on the right planet, you can just dig around on planets with high concentrations of the ore you’re looking for if you’re just looking for specific metal ore.
Exoplanets spawn randomly whenever the devs want them to. Once they ‘spawn’, they last a few days and can be accessed by warping to them from the closest planet they orbit (will require full points into warp distance epic). Usually me and @Soju-VB sets up free location tokens for them and 99c atlases within a few hours, you can pick them up to use as a warp destination if you don’t want to bother waiting around searching the sky to get a warp destination on them within the few minute timeframe they appear. T7 exos also have the three lucent gems, but they’re usually gone within the first three hours.
Not really worth imo as tools/weapons. More useful as expensive building blocks. ¯ \__(ツ)_/¯
There’s a farm school that shows you how to farm all the crops(minus goo, that’s complex and dependent on what you want). There’s a few portals there, but I only can remember there’s one at EZPZ hub on Maryx.
As for location, purely up to you. Just make sure cuttles can’t fly in if you’re on T3+ planets 
Generally selling to request baskets or from a shop stand will yield more coins than a minter. You can check prices from the BUTT tool for at what price people buy and sell things at, and on which planet. You can go to that planet after looking it up, and find it in the knowledge tab to get a waypoint there… or hopefully find a portal there. As for items that get you decent coins, things that are hard to farm generally sell well. Hunting loots, beans, leaves ect. Mining and selling gems get you a lot of coins as well.
There is a very good guide on forging to get started with, tho you will want to ask around and work out better ways to forge yourself if you want to go all the way into forging. It is also perfectly fine to avoid it like the plague like some people as it will seriously drain your sanity with bs RNG once you start forging more complex stuff, and forged goods are a lot cheaper nowdays. Most tools will net you far more coins than the price it cost to buy them. However, if you do decide to try forging, I strongly recommend trying it out on either the forge simulator or in a creative world (there’s a public one in TNT megahub, Janus II) before trying it with actual materials, as forge ingredients are NOT CHEAP, nor is setting up a proper forge with coils. Worst thing that can happen is dumping a few hundred thousand coins to get a full forge running to find out you don’t like it one bit.
That is entirely up to you. There are people that do like the solitude on their own planets. Just remember you will have to pay real money to spawn and fuel your own sov planet.
I would recommend against making a shop on one tho, quite a few people tend to ignore shops on sov planets as it is often a huge pain in the ass to find a way there. Only if you get good connections to portal hubs and advertise it a lot.