You say Russia viewed Trump as strong and that's why they didn't move in on Ukraine.
I have another avenue of thought that I don't expect you will agree with but I'll share anyways:
- Trump wanted to dismantle NATO, which would help Putin
- Trump wanted to bring Russia back to G7, which would help Putin
- Trump withdrew from Open Skies surveillance treaty, which helped Putin (which he did after he lost the election)
- Trump dismissed and cast doubt about Russian hacking, particularly when the U.S determined that Russia hacked the DNC in 2016, while ironically enough, he encouraged Russian cyber attacks on national TV saying, "Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing,"
- When addressing Russian election interference and cyber attacks, Trump proclaimed "I don't see any reason why it would be Russia" after speaking directly with Putin, defending Russia and trusting Putin over our own intelligence agencies. Later he "corrected" himself, claiming that he meant to say "wouldn't" instead of would.
- Trump suggested the U.S. work directly with Russia on cybersecurity. Trump refused to punish or critique the SolarWinds hack.
- Almost directly after the 2016 election, Trump sought to weaken U.S. sanctions on Russia, while he was even open to lifting sanctions.
To my eye, Putin was maneuvering Trump to enact all the measures he needed to make an attack on Ukraine possible. Trump was either too stupid to see it, or wholly complicit. My belief is it's the former, but I'm sure there are some who think the latter.