The Midgame
The midgame in FrogComPos Band is characterized mainly by trying to cover your resistances while still doing enough damage to be able to kill monsters. You'll also want to pick up as much speed as possible. But the main thing you'll be doing is exploring. While in town, go ahead and hit that < key and take a good long look at the wider world around you.
You'll notice other towns, dungeons and paths between cities. For now, try and stay on the path to reduce the chances of getting ambushed by random monsters. It might not be much more than annoying now, but later on in different zones the enemies can quickly ramp up in difficulty. For now, head to Anambar, the city in the northwest, and down to the troll / orc caves just southeast of it.
If you get stuck there, try some of the other dungeons around your level. Try to pick up useful detection staffs and rods, especially detect monster and detect traps. Keep potions of cure critical wounds on you at all times, just like teleport scrolls. Do the various quests found in different towns throughout the world. Be aware that these quests are usually quite a bit harder than the danger level indicates, especially the Cloning Pits quest.
Once you've got the cash flow, you can teleport between towns you've visited by using the option in the inns. This can be helpful when you're flush on dosh and would like to upgrade some equipment. Take a shopping trip to every city’s black market and you might find a handy ring or stat potion. Perhaps a staff of Enlightenment to map the dungeon for you? As you level up, the black market will offer better and better quality items to purchase.
You'll be towards the back end of the midgame when you're going through different dungeons, hunting dragons and other bigger monsters for their tasty item drops. Your resists will start to look good; you'll have something close to double coverage of your base resists and decent single coverage of some of the high resists. You’ll start swapping different sets of equipment in to try and get better combinations that will let you do more damage or have better resists. You’ll start thinking about diving down Angband to finish off the final bosses of the game.
The Endgame
I can’t give too much advice on the endgame, having only gotten there a handful of times myself, but in general, be a coward. Detect everything as thoroughly as you can before ever entering a room. Kill every weak enemy you can for exp and use every cheesy strategy you can come up with. Dig holes in walls to draw out powerful monsters and fight them one on one. If you’re an archer, use scrolls of phase door to bounce around once a monster gets into melee range with you. Use every advantage at your disposal, because once you’re in Angband facing down monsters that breathe multiple elements simultaneously, can stop time, and summon enemies that then summon more enemies, you’ll wish you had run practiced running away earlier.
In general, keep more items in your inventory than you think you'll need. When you have more than 300 HP, start carrying around potions of Healing for emergencies. Speaking of Healing and Healing potions, you'll want to hoard all you can of these to prepare for the final fight. Use them if you need to, it's stupid to die with an inventory full of healing potions, but keep as many as you can for later.
Check out the Angband ladder for FrogComPosBand https://angband.live/ladder/ladder-browse.php?v=FrogComposband&r=&c=&n=&e=&s=0, especially other characters of your class. Read spoilers on monster levels, spells, anything you can find.
Advice for quests found in towns: https://pastebin.com/ZLZZz45j
Demigod mutations: https://pastebin.com/hTi24Nky
Arena rewards and various other small spoilers: http://nikheizen.github.io/pages/rewards.html
Dungeons, dungeon guardians and quests: https://pastebin.com/AVsp31k8
One last bit of advice, maybe try the Munchkin personality if you get stuck in a rut. It gives huge boosts to your stats, makes it easier to level up, and starts you with a million gold. You can't really get credit for beating the game using this mode, but it is great for trying new character combos and learning how places you've never been work. It's worth checking out at least once, especially if you're learning the game.