A few tips from those days, which I remember well!
Bully/pizzle sticks are brilliant chews for pup. Completely natural and last for ages. Some of them stink, but the ones I get from Amazon (called Best Bully Sticks, I think) hardly smell at all.
To get them to stay longer in a sit-stay or a down, you have to build up the time very gradually and methodically. Google the 300 Peck method, where you add literally one second at a time, and start over from zero if they break the sit. It works really well, it makes a good game, and they do learn! I am very specific about adding 'stay' to a sit command. 'Wait' just means 'hang on a sec' but stay means stay. We haven't got it perfect, but if you start now, you'll have it down in no time.
You can also teach them to 'go to your mat' when you a teaching a longer stay - I have a mat or bed in each room where if I send him there, he has to lie down and be calm. You can start this when they are really young. Again, you have to build up the time they stay very gradually.
They are much worse when they are teething, and there's not much you can do about that except grin and bear it and know it does pass, honestly!