Security guarantees like the Budapest Memorandum? https://en.wikipedia.org/wiki/Budapest_Memorandum

Worthless.

Note: Budapest memorandum is:

1. Not binding; 2. Non-actionable; 3. Never broken by the western countries.

Budapest memorandum is tiny and clear. Go read it and decide for yourself whether US has broken it or not.

NATO Art. 5 guarantees are binding for all NATO members, out of which there are multiple that would be willing to engage.

While the proposed replacement, art.5-style guarantees backed by the US alone, is guaranteed to fail, because there is nothing in the whole world that can enforce and bind US to actually act on the signed document. Current administration can just basically say "fuck you lol" once russia attacks again and that would be it.