1) My theory about Trump/Republicans is that their boisterous pro-military stance discouraged Putin.
-
2) Biden looks senile, or at least weak.
-
3) While an unpopular idea, the US could have offered peacekeeping troops to the civil war in Ukraine that immediately preceded the Russian invasion.
These are actually pretty interesting points, but I don't think the argument is very strong.
In basic, your argument looks something like this:
Trump was a strong, militaristic leader;
Putin feared a strong militaristic US;
Biden is not militaristic, or appears not to be.
If Putin feared the US, he wouldn't have invaded Ukraine.
Putin feared Trump, thus didn't invade. He did not fear Biden, so he did.
I'd say it breaks down in a few places: Trump was also the president who didn't start new wars, and leaned on that in his re-election campaign. It is equally reasonable that Putin would have invaded knowing that Trump would focus on his re-election.
There is also a lot of suspicious behavior between Trump and Putin, which could equally explain the lack of action from Putin, although this would be much stronger if Putin invaded the day Trump left office.
Biden/Democrats appear to be "anti-war" in the US, but in actuality have been very militaristic. I doubt Putin is informed by American media, and would know that US troops are more mobile under Dems.
Biden does look weak, which could mean a lot for an authoritarian like Putin, and I would be interested to see internal reports from the Kremlin on this topic specifically.