Honestly I just sell materials if I don't have enough from the quests. I've so far started with a magic user in the first run, and a worker in the second run. Fairy race for both runs (didn't realize at the time that other races also had hidden classes). In the first one I got some of the most cursed quests, for example my mage guild would only offer me aphrodisiac quests, my fighters guild only wanted me to full clear dungeons with the occasional slay x amount of bandits, my servants guild just spam asked me for permanent workers instead of the 3 day lease variety, and my worker guild wanted increasingly drastic amounts of food. In that run I actually had to go through I think 3? debts before getting the points needed. Didn't help that I didn't really know what I was doing either. 2nd run I only had to do 1 payment and got some fairly blessed early quests, it was overall significantly easier.
A few pieces of advise: Do not hire poor growth workers to keep, only to fill quests at best. Always aim for at least good on the ones you are going to keep and always check all the guilds as well for recruits. Avoid trying to clear out dungeons before you are ready, it can be incredibly expensive time wise if you end up dying. Make sure no workers are idle, always have them chopping wood, mining stone or iron, or at the very least getting food. Even if you don't use the materials they harvest, you can sell them to make up whatever deficit you have come debt time. Also use the "date" function every 24 hours to fill loyalty, or train workers for quests.
Oh and as for upgrades, only do tailoring and smithing until you get the debt taken care of. Otherwise you starve yourself for materials and waste a lot of time farming them. Doing it this way in my 2nd run got me through the debt phase on like day 21ish I believe.
Edit: Forgot to mention how I got through Dungeon / Slaying quests. Either your main character or 1st servant need to be some form of combat class obviously. I usually side with the fighters guild first so I can get the sword and armor, then go archer class on the 1st servant to get wind flurry which hits 2 enemies at 0 cost, you can use it with any weapon. Then I'd send my fairy master in with the 1st servant cause he gets access to the once per day fairy heal ability that heals 50% hp. That will easily get you through any slay quests, for the Clear Dungeon ones, you need either your Master to be a healer or have a 2nd servant thats some form of combat class. If you need any extra classes, make sure you use the 'Date' option every chance you can as well as clicking 2 people for some bedroom fun, you can also get exp through intercourse in this after all.
Edit 2: So I initially thought prostitution was a red herring job early, but then I tried a 'challenge' run more or less where I ONLY use prostitution to get out of the debt, Master was a Rabbit, first servant was a Fox. And....on day 3 I had the 1k I needed for the first debt. I imagine by the time I reach the deadline, the 14th day, I'll already have enough for the 3rd loan payment. The only downside to this is that there are very few guild tasks my mansion full of prostitutes can do. And ofc my first 4 tasks were all of the "go slay some wolves" variety. So its probably still gonna take me a while to move past the debt phase, but I doubt I will ever have much trouble paying.