- Why don't they carpet bomb all of the Ukraine?

Russia does not have anywhere near enough aircraft to do that. If they tried, they'd lose more of their air force.[1] Russia does not have much of an aircraft industry left. For one thing, much of the USSR's aircraft industry was in Ukraine.

- Why not fire off a litany of missiles?

Nuclear? Russia so far has not wanted to cross that threshold and start WWIII. Non-nuclear, they have to build them.

- Why not conscript the bulk of their manpower reserves and actually invade in earnest?

Because Russia is running out of manpower reserves. The draft in Russia now covers all males from 18 to 30. In that age range, men can no longer leave Russia. Russia has 1.4 million military casualties so far, with about 400,000 deaths. Currently, the casualty rate is higher than the new recruit rate. Conscript soldiers serve for one year, but there is heavy pressure to sign up as a "volunteer" with no time limit. The Russian tradition of throwing recruits into battle with a huge casualty rate continues. It worked in WWII, but cost 20 million lives. It hasn't been working in Ukraine. The Ukrainian claim is that the survival time for a new soldier on the Russian front lines is four hours. This is probably exaggerated.

It was policy for a while that the draft wasn't enforced too strictly in Moscow and St. Petersburg. This was to avoid generating political opposition to the war. That seems to be over.

[1] https://nationalsecurityjournal.org/putin-cant-fix-this-the-...