There's a rail connection between anywhere and anywhere with a station, if you're prepared to change enough times! Please do tell me how you get from Bromley South to East Croydon by rail with one change at Beckenham Junction, as you imply. If you said by changing to Tramlink at BJ, okay, but it would cost an awful lot more and, as you say, the SE Rail service has deteriorated to half hourly when many are shopping. I also doubt that the journey time would compare with the SL5 unless each train and tram met each other perfectly.
I admit I haven't been there for about four years, but five hours in the Whitgift car park cost me £10 then, and I can't imagine it hasn't increased, particularly as the local residents have had the misfortune of one of the most toxic council leaderships in the country for many years past.
I think, by the way, you'll find it's the 227 bus route, as it was when I lived in Bromley, worked in Beckenham and used it on a daily basis back in the 1960s. All in all, you don't seem to have a familiarity with the area.