Unfortunately, the best 921 LED retrofit bulb, the
Philips 12841B2, was discontinued a few years ago in favor of a cheaper new version.
It's difficult to replicate the output produced by a filament, and harnessed by a reflector, using a bunch of LED emitters.
The designs that are effective, albeit not universally -- it depends on the fixture design -- are like that Philips bulb. They utilize a few high-quality emitters that have their output directed at the reflector, which does the work, as with an old incandescent bulb. Not a bunch of cheap SMD emitters that rely on ambient radiation to illuminate, leaning heavily on quantity over quality.
The most common attempts, using a phalanx of emitters resembling a disco ball, attempt to overcome that through brute force, and don't work well the majority of the time. Those that are designed and made cheaply are even worse. More so when sold at marked up prices justified by hollow marketing. If one wants to experiment with them anyway, just order them from AliExpress at a much more realistic cost.
Also unfortunate is that the reputable brands, like Osram/Sylvania, and Philips have dialed down their efforts from before (that 921 is just one example), and moved away from the good designs in favor of cheaper, less effective ones.
And keep in mind that, while the crappy LED retrofits are mostly harmless when used for things like general/interior lighting, using them in functional and signalling applications -- brake, reverse, signals, etc. -- increases risk. It's clearly evident when stopped behind a vehicle with such retrofits, and all that emanates from the lights is a dim glow, and/or a rapid flash, which the owner/driver is oblivious to.
There are potential benefits with general applications -- a 3W LED in that bed light is going to drain the battery much more slowly than an 18W 921 -- but if there are no good solutions, sometimes it's best to stick to the tried-and-true that is known to work.