Not with motivation, that thing is rather fleeting and unstable. Though not only forcing it with discipline, that's a rather limited resource as well.
Basically using as little discipline as possible to slowly form a habit.
So now you're going at least once a week, good work. Keep it going until it becomes a habit, few weeks to months. Once that has become a habit, add in another day and repeat the habit forming process.
It took me nearly a year to go from once a week to 5 times a week and by now during winter i can add in cardio sessions as well and up the frequency easily to 7 days a week.
Back extensions are probably the best ones as those directly train lower back muscles with full range of motion, but any deadlift variation, good morning and rows without chest support do train lower back in isometric holds as well.You're not wrong, squats are mainly for legs, but as those require bracing it trains core muscles as well and lower back falls under core muscles too, though it's questionable how effectively it trains lower back muscles in comparison to direct work.