Bringing those breakways territorys in to Russia, will only erode Russia. None has a Russian majority population. There is no enthusiasm fom the part of the Russia population on that as well. Besides the Russia budget can't cope with such an effort.
Both Abkazhia and South Ossetia should give away some villages to Georgia, in exchange of its independence recongniztion from Georgia itself. Shoould they join Russia? Not at all.
Ukraine, will be an EU member in the future. Given the chhange they won't thinl twice.
There isn't any choice when it comes to the Ukraine. It's a permanently hostile ultra-nationalist state otherwise, with neither its regime nor its Western backers in any mind to do a deal with Russia as the failure of the Minsk agreements over 8 years, and then the Istanbul talks have shown. So it will be incorporated into Russia or made into some sort of dependency with nominal independence.
Pridnestrovie is very much pro-Russian, even if the population is ethnically split between Russians, Ukrainians and Moldovans 3 ways. It would love to become part of Russia. But it will accept merging back into Moldova too if Moldova adopts a pro-Russian trajectory, so that's an option as well.
South Ossetia - wants nothing more than to join Russia and merge into North Ossetia as a single federal subject. I don't know if it's the best idea strategically, especially as Georgia is making a genuine effort to stick to neutrality over the crisis - but then if Russia is going for it then its going for it. If it's a pressure tactic against Georgia than it has probably gone too far already, they're gearing up for a referendum on joining Russia in June over in South Ossetia.
Abkhazia won't join Russia as a federal subject, they have no mind to do that. They can create a union with Georgia in the future maybe, or stay independent.
The Russian budget is not going to be any better off paying for all these territories as things are anyway; they're already dependent on the Russian budget as things stand.