The partial mobilization is a band-aid; it won't make the Russian army effective (let alone the air force, or the navy). I suspect these guys are just being thrown in at the deep end. They're doing to die, and not do very much in the process. The main goal is to keep something in the field, so the Ukrainians don't just walk to the border because all the short-term contract troops have gone home.
Building a real army takes at least a year; the Ukrainian are working on this, via mass training programs in European countries, and here in the Ukraine. The guy who organizes my AirBnB is in an infantry training programme; when he picks up the rent, I give him a huge shopping back stuffed with chocolate for his unit.
Then there's the question of equipping and supplying that army. Ukraine has the arsenals of the West behind it, and ultimately the military strength of a country is its economic strength. Putin's underestimation of the power of the West is as great as that power is proving to be.
I've no idea if Russia is also working on property training new units. It seems somewhat questionable - training in Russia is performed by the stay-at-home batallion (each regiment is three batallions, two are normally deployed, one stays at home), but a lot of those stays-at-home batallions have been deployed. If you just throw people in at the deep end, if they do survive, they have experience, but little training. No good for more complex operations, such as offensives.
However, the troops anyway to my eye have no motivation for offensives. Some units are just brutual thugs, killers, torturers and maimers; others are conscripts, and the army as a whole knows perfectly well they were thoroughly lied to and sent into the battle in the worst possible ways, tactically and strategically, and are paying with their lives.
This is why there's so much reliance on artillery; it just sits there, behind your lines, so you can control it, and the men firing the pieces, and all they have to do is put shells in a gun and fire.
I don't think Putin will use a tactical nuclear weapon, because it just won't do very much to the battle; they are surprisingly ineffective on dispersed troops. Even to use one will turn the world against him, and he would have to use a lot. Putin would have to build a new army from scratch, with so much strength it could overwhelm the Ukraine supported by the West, and that is not possible.
I hate being in a situation where I have to even think about whether or a nuclear weapon will be used. Putin has a lot to answer for.