Would salah behind an imam who says the takbir after completely moving into the next position be valid? A lot of imams around me do this so would it be allowed for me to pray behind them from time to time or must I look for a new masjid to pray at?

Ustadh AbdulAziz al-Haqqan

Since I know that you suffer from obsessive thoughts, disregard all of these thoughts altogether so that you don't open a door for obsessive thoughts to enter.