I'd say it's almost entirely that "rightful sphere of influence" stuff they've been going on and on for decades. After all, before the war even started, NATO tried negotiating with them and the Russian demand for calming things down was NATO going back to its 1992 borders
. Which, obviously, was completely unacceptable to NATO since none of the newer members would accept it, not to mention that the older members would look like complete wimps on the global stage, and frankly, I honestly don't think anyone in the Kremlin believed it would be done either.
To be fair, they seem to have stopped talking about this a bit and started focusing just on Ukraine, Belarus and Central Asia as they confront the limits of their own power, but that was at least the rationale they were using before and at the very start of the war.
Apart from that, there's also, as you've mentioned, the fact that there's nothing but flat terrain from Ukraine to Russia, and that probably also plays a role, but I doubt it's even all that important tbh. With how close the Baltics are to St. Petersburg, the city is pretty much destined to immediately become besieged in any war with NATO, and from there it's a pretty comfy way to Moscow given that the two cities have probably the most extensive infrastructure connecting them in the entire country.
Not to mention that the Donbass front has basically not moved at all and there were even some reports of Ukrainians recapturing some pieces of territory they haven't held since 2014, so, if nothing changes and Russians won't get anything else except the land bridge to Crimea, which they are now claiming is the only thing they wanted anyway, nothing changes with this situation, and NATO will still have the possibility of just rushing Donbass, breaking throught and entering Russia. Even if Ukrainians surrender the entire Donbass to get peace, it's a tiny sliver of land that NATO won't have any real problem getting through, so this is ultimately quite questionable how much it improves Russian defenses from a strategic perspective.
For now, all they seem to want to do is make the international trade they participate in be conducted mostly in the Yuan. Rather than trying to be the unquestioned world leader, they are focusing on creating their own currency empire around themselves, involving the countries they get their imports from, where most of the trade is done in the Yuan. Maybe they have ambitions to eventually completely overtake the USD, but that would still have to be a long-term, multi-decade objective.
April-June figure driven by capital market liberalization and Russian trade
asia.nikkei.com
To be completely honest that's much better than I've excepted. The US-EU trade is one of the arteries of world economy, matching half of it while not trading with either of them is a pretty significant achievement. Not to mention that intra-BRICS trade has only really gotten a real shot in the arm with the start of the war, it was growing even before that for sure, but the war really sped things up, while the US-EU trade had been developing for decades.