I can only really go by my own experience with Blondi, which does sound startlingly similar to your own in many ways, such as time of ownership and lack of any discpline on arrival. That's not to say that my way is the right way, but it's just what's working for Blondi and I.
When we got her, I gave her two to three weeks off completely to settle in. All I knew about her at that stage was from the past owner: pulling on leash, poorly house trained, can't be around other animals, and, as already stated, only has rudimentary sit recognition.
After about three weeks, I tried her on the leash outdoors to see how bad things really were for myself. It was bad -- very bad. I came back home and my arms physically hurt from being pulled. I figured, at that stage, to be fair to all, that I wouldn't walk her again until I felt more comfortable and that she wouldn't get a bad name for herself by scaring all other dog walkers with her lack of discipline.
For the next couple of weeks, I walked her up and down, up and down, round and round the garden on the leash for about twenty minutes twice a day. After that time, I took her to the front of the house and walked her around the forecourt where we live. She still pulled like a nutter, even at this stage, as it was new smells, sounds, and experiences.
By this time I was feeling a little bad that she wasn't getting the walking time needed; not only for the exercise but for the healthy one-to-one bonding which takes place during a walk. So I purchased a Halti. She literally ate through that in under five minutes. Back to the pet-shop it was. This time I purchased a
Mikki Harness. The difference was instant. Of course the dog still pulls but it stops you feeling it. This was a major step forward as it enabled me to at least walk her without feeling physical discomfort.
She has been wearing that since, but during the past two weeks there has been a lot more loose-leash time during the walk where there is quite a loop in the leash meaning she's not even trying to pull. (Just my luck that she's now come into season so no walks now for three weeks or so!) Once I'm happy that the leash is remaining loose, I'll remove the harness from the equation. I'm in no hurry, as, after all, I'm trying to undo 18 months of habit that she's had.
As for training time - during the hot and humid weather we've been having, I felt that the dog's attention has been elsewhere for obvious reasons, and of course I haven't felt great in it either, so neither of us would have been giving our best. I decided not to continue outdoor training during this time.
However, I still kept recalling her to me indoors at random times and asked her to lie-down, sit, etc. I maybe do this three or four times a day.
Usually, weather being normal, I train her oudoors in the rear garden twice a day. Once in the morning and once in the evening. Both sessions last for about ten minutes each (or until my pocket full of treats is emptied), usually sit-stay, down-stay, and recall. After training in the evening, I get her favourite ball out of the cupboard (I keep most toys so they retain their "value" to the dog.) and we'll play "fetch and here", make her sit and look at me in the eyes before I throw the ball for her when she returns. (Stealth training as I call it -- wrapping training in play).
Walking wise, I walk her twice a day, each lasting about 30 to 45 minutes. I vary the routes but only because I want to get her accustomed to as many scenarios as possible, such as heavy-traffic, heavy people density, quiet, etc.
Now, a few weeks on, she will sit on every request, lie-down nine requests out of ten, no longer chews our cushions, recalls in quiet spots though still very hit and miss when distracted, is fully house-trained, and seems to be as happy as a pig in mud.
During the day, as I work from home, she can come and go in and out to the rear-garden as she chooses. Invariably, she chooses to lie down on the landing or another cool spot.
Not sure how useful any of the above is but if it helps in any way, great. I'm a firm believer in letting dogs have their own time and space, keeping training to short and sweet burts, and hiding as much of the training as possible in play activities.
I think, going by your original message, that even pushing the dog out of the equation for a moment, you are starting to feel a little frustration, and so a break will do all parties the power of good. Time for a recharge. I do it whenever something is taking longer than I expect. It gives me time to see all the things that she is doing that she couldn't a few weeks ago., A great confidence booster!
Oops. I should add, I still don't let her off the leash in open-spaces yet as I don't have 100% confidence that she'll come back when called. But I'm confident that the time will come, and neither of us are in a race to get to the winning line. Just enjoying the incremental steps to the goal.