Main question about my plan is it safe or not keeping usdt for months?
Let me answer your main question first: Yeah, tether is a centralized cryptocurency which means it may get seized by government at any time for whatever reasons and most probable reason must be money laundering prevention. Hence, I will not advice anyone to go holding tether even for weeks.
I think I will sell everything I have (btc and alts) in the end of december, to USDT.
December is going to end in 3 days and for some countries, in just 2 days but we have not seen FOMO which must be the last phase of all bullish market hence if you book profits now in USDT then you might miss upcoming bull market. I guess you must wait 4 to 6 more months for FOMO and then you may decide.