I managed to finish Inferno Mode (20 days Repayment, 2 Million Gold) at Day 16. Plus all other "main quests" required to trigger the end game/ "true ending".
To make things clear as the game itself states the number of days for payment does not fcking matter, whether you select Saint Mode (30 days Repayment, 1 Million Gold), or Relaxed Mode (Unli days, 1 Million Gold). The # of days time limit is more of a self imposed challenge/ gameplay modifier that the player can adjust based on how much they want to go full try hard on a H game (well for this game I went full sweat lord try hard). It does not change anything aside from failing the time limit (there are no CG to failing, just few dialogues and you get to restart to "NG+" with all your stats + items + money, or reload from previous save. It also does not provide any unique title/ achievements/ items (aside from self accomplishment lol).
However.. Based on the game's narrative, the canon scenario most likely is Inferno Mode (because berserker is a jackass), and Enicia went full multi Job juggling to gather 2 Million in 20 days lol.
So as others already stated on the forums here is a guide on how to complete inferno mode repayment (20 days, 2 Million). Easy way is going Full H Playthrough (downside would be a "bad end" for Enicia). The way I did it and the more satisfying way is the "pure"/virgin playthrough (try hard gaming).
Guide:
Once you get free roam, always get the exp bonus at the altar (sometimes lust reduction if you are playing pure route, and have accumulated some unintended lust points), always do the Pig finding daily quest from the girl near the library, study sacred arts at library, healing patients at the hospital (once a day), spirits purification at knight girl.
Recruit the 2 adventurer asap (the ranger girl and the warrior perv dude).
Always be on full 3 man party. When leveling just level their primary stat (Enicia = Spirit, Thief/ranger girl = Skill, Warrior dude = STR).
Once you've accumulated enough gold and you really hate yourself/ would like to succeed in a pure route/ make Enicia have a happy ending, you will buy a fishing rod and start fishing and love it. The minigame is so chill and Enicia seems so happy fishing (plus the BGM is nice). Your priorities when buying are 50 Baits per day (min max fishing per day), then you buy the 25k item that increases the critical range when fishing (don't buy the auto fishing one they do not stack and that will not allow you to crit, thus lowering your income in exchange for automation Crit = 2 fish instead of one). After getting 50 fishes (achievable in one day) the fisherman dude will give you an accessory that increases chances of rare fish and + 1 luck (equip it when fishing along with the critical mark increase when fishing).
If you have further free time, go and "help" church visitors so that your church stat goes up and more donations (the donations is shit, but you see numbers go up and that's good enough for me). About church level 2 (or 1, did not check) the confession booth will open up and you can do that with extra stats at night.
Day 2-3 You need to start also including the 10 Floors dungeon into your daily task (it can only be entered once a day). So in total if you want to clear 10 floors, you would need 11 days in total (9th floor would require 2 days on the first entry because of way that floor is designed).
Aside from the daily task that you will be cycling through, there will be "main story" quests that you will need to complete for more money/ main story progress (not required in repayment but some with monetary rewards will help). The lord questline will open as you keep purifying ghosts (this gives lots of money), the adventurer questline (it starts after you are asked if you want to purchase license), Wize questline (starts day 7), Director old lady questline (no monetary reward, plus it will cost you 80000 + 2000-6000 (depending on how many revive item you got) to do (I did this and still have enough money by Day 16. Plus some other main story quests that I may have forgotten to list (they are quests with star near the quest name).
The main quests you need to do are: explore the 10 Floor dungeon (you get 1 million reward from this, not counting the other valuables you may obtain from the dungeon), and the lord questline (a comment from before said you can get up to 300K from this questline, but I have not personally counted). I got some of my gold from fishing (it's about 50 HP cost per day, 500 daily bait cost, your actual time Fishing COST, but it does not change the time in game, about 30 - 40K per day).
Don't forget to min max with foods too. Best food per price is the crab (plus it unlocks a trait that Enicia likes crabs), and the wine is the one where you unlock from Wize once you complete her quest on day 7 (30 SP restore wine).
So yeah here is the TLDR daily try hard routine: get exp buff from altar, Pig quest, library sacred art, do the healing (until your sacred art is level 5), explore the 10 floors dungeon, fish 50 times per day, purify ghosts (until you get max/level 5 lord quest rank), talk to church goers, (optional do the star/ "main quests" if you have free time), do confessions at night.