I’d take it to someone decent and get it checked, you’re right that does sound like early torque converter. My bm did a torque converter but the m57 engine does break converters/boxes worse than any other. My mates bounces intermittently on lower end 1050-1500rpm and has done for 40k. If it were me I wouldn’t worry as these don’t sound like they do boxes or converters as a rule. Get the oil flushed, it won’t get all the oil out of the converter but it’ll prolong it for a very long time I would have throught. As for the stuck in gear, mine held in second longer than it does now pre getting the flush done and goes through the gears perfect now.
Tom