What is the optimal trophy level for farming at a level 8 town hall?

It is SUPER easy to make a lot of Gold/Elixir and a modest amount of Dark ( you won't be rolling in it, but it will build up slowly )

Trophy Range, Stay between 1900 -> 2100. This is Crystal III, with 35k bonus of Gold/Elixir and 100 Dark.

Basically Town Hall Snipe your way to this trophy range ( attack bases with town halls on the side with a few archers, but have an army to back them up just in case they have hidden teslas, etc). Once you get to 2000, you don't want to lose your Crystal III status, which happens if you go below 1900.

At this trophy range you'll find a good supply of side town halls, and people loaded with loot. at th9 and th10, it is very hard to protect all the collectors, so they are often on the outside of the walls, covered by archer towers and canons.

Your strategy is to snipe town halls whenever you can, getting your 35k bonuses. But to also find bases where the collectors are full ( learn to recognize the inside of elixir collectors full level, and the gold has a box outside that fills with coins ) and compare it to how much is in storage. If they have 300k available and their storage looks low, and the collectors look full. Its time to attack! Doesn't matter if its a side town hall or not. You don't care about getting the win, just the loot. Time to Attack!

for Attacking we are going to go with BAM - Barbs, Archers, Minions. Super CHEAP to train, super FAST to train. every single barracks will be producing units very quickly. Don't worry about spells too much, but a rage spell on hand can be useful. Both Dark Barracks queue up minions, 2 barracks dedicated to barbs and 2 dedicated to archers. Just keep them constantly queued up. The ratios between troops doesn't matter too much.

Now the trick is, use as few troops as possible... for collectors on the outside, put just enough barbs in to take them down, for collectors over the wall, use archers, and once they are getting used up and there is still more loot, use minions. Once you've taken down the collectors, end your battle. Don't try for storage unless its placed in an unprotected place. If you only use 80 - 100 troops to do this, you'll be ready to find another battle straight away after this attack, and because this loadout trains fast, you almost never run out of troops unless you find a super juicy base that you throw all your troops at.

Your barb king ( and queen if you are at TH9) are just there as backup or to help with sniping THs.

You can EASILY make 150k to 300k doing this. You will lose trophys, but just snipe your way back to 2000. In an hour or two you can make several million.

This works well for people at TH8 and TH9. You will make so much loot that most of the time your builders/lab will be busy and you will have nothing to spend it on.

This is a good time to take a break from farming and practice your war attacks, queue up some expensive troops ( you are rich now! you don't care about making a profit on these attacks ). Try out All Drags, or GoWipe, and experiment with all kinds of combos. Then when you drain your Gold/Elixir a bit, go back to your farming strategy.

You should farm at about 1200. It will mostly have Town Hall 6 to 7 which should be a solid place for farming. It has a decent amount of loot too.

The answer will largely depend on what elixir/dark elixir you're willing to expend and what attack strategy you are planning to utilize.

Personally, I'm attacking with a Giant/Archer/Barbarian/Breakers army composition with 3 healing spells in support. I sit in the 1450-1650 trophy range. However, there are certainly other options.

BARCH is good for bases with full collectors or that are poorly designed. These can appear at any trophy range, but tend to appear more often at lower trophy levels. Sadly, I've found that these opportunities aren't as numerous or profitable at TH8 compared to TH7.

More robust attack strategies (all dragon, GOWIPE, etc) allow for higher trophy ranges and more damaging attacks, but also cost more to launch and take longer to recover from. But with most bases with ample loot now being well defended, this can still be quite profitable, especially if you're specifically looking for some resources that aren't used to rebuild your army and spells.

Trophy hunting is another tactic where the goal is to score 1 star and collect the loot bonuses. The goal is to to search for and attack as many bases as possible that are poorly designed and your attack can easily yield 50% destruction, and perhaps have a Town Hall that you can easily reach. Some drawbacks to this tactic is that you will have to climb the trophy ladder in order to score really good bonuses, which could bring you out of your sweet spot when it comes to looting the rest of the base, and expose you to higher-level players that would be more capable of obliterating your base in a raid.