Hey Dominik, nice post
Good luck with your trip!
I too put a few feelers out for information about that Far Eastern part of Russia, but I didn't get much. I think everyone either races through to / from Vladivostok, or heads up to the Kolyma Highway or maybe BAM.
I'm heading that way in a couple of months, and I plan to loop around the region, taking the main highway down to Vladivostok, then sticking to smaller roads close to or on the coastline to loop back to the main road. The region looks hilly and beautiful with a lot of deciduous forest (which is not all that common in Russia) where tigers still roam (though I have no expectations to see one...).
Khabarovsk seems like a nice place to break a trip, and there are some petroglyphs fairly nearby. The Russians are also building a new cosmodrome near Khabarvosk, might see something interesting from the road?
From there I think there is not much to see on the main road; I plan to stop in Birobidzhan which is the capital of an odd, Jewish republic of Russia, and then Blagoveshchensk where you can look across the Amur River into China.
After that there is Nerchinsk, which is interesting probably only if you are really interested in history (Google the Treaty of Nerchinsk if you are interested).
West of Nerchinsk are places I have already been to...
Chita has some nice streets, and Decembrist monuments.
Something I would absolutely not miss is the Datsan (Buddhist Monastery) at Tsugol, which is a magnificent sight; one of only two I think in Russia which survived Stalin's ruthless persecution of religious paraphernalia. The other is nearby in Aginskoye and is also worth seeing (it is more active), though Tsugol is the gem.
The backroads of Buryatia are wonderful, especially the Barguzin Valley just east of Lake Baikal. I went in September when the larches were turning and it is one of the most beautiful places I have ever been. There are nice Baikal beaches also just south of Ust Barguzin.
Ulan Ude is not a bad place to stop for a while, with some nice Tsarist-era houses in the centre. Then I suppose you will enter Mongolia from Kyakhta.
Alternatively, you could enter Mongolia further east at Solovyevsk which is a slightly more interesting way to enter Mongolia IMO as you make a pretty abrupt transition from the rolling hills of the Trans-Baikal region to the endless grassy plains of eastern Mongolia. On the other hand, you will probably miss Baikal, which is also worth seeing.
This page of my website describes a trip I made in 2010 starting from Kyakhta (coming out of Mongolia), visiting Ulan Ude, the Barguzin Valley, Chita, Aginskoye and Tsugol datsans, and then heading back into Mongolia in Solovyevsk
https://eurasiaoverland.com/2010/10/10/23i/
One thing to be careful of in Russia is border zones, which are generally anywhere within 15 to 25 km of an international border. On roads they are clearly marked with a blue sign labelled 'Warning: Zone of the Border Control'. Do not pass these; the penalty is deportation, a fine (not so much) and a 3-5 year ban from Russia. This includes the border town of Kyakhta apparently; a Swiss biker was recently caught there and later refused re-entry into Russia. His bike is still there... So, if you will cross the border at Kyakhta, I recommend you go straight to the border crossing; do not leave the main road! (I had a look round Kyakhta and had no issues, but getting away without being caught does not mean it is legal...).
Will be interested to see if anyone else has some tips of the Russian Far East,
Hope your bike arrives soon
EO