I assumed its sri to be 180 degrees which would make it ~2000lm total
Given your assumptions ≈2,000 lm would be correct. What you do not have is the efficacy (lm/w) and you did not include the losses due to current regulation or light bulb diffuser transmission loss.
which states an approximate 2.1cd of luminous intensity for its
combined RGB output.
Not sure where you got 2.1 cd.
Assuming I get 5m of 30LED/m strip, that would be 315cd total.
Using the average cd values I get 3.05 cd (0.900 + 1.250 + 0.300) per LED. 150 LEDs would then be 457.5 cd.
The problem here is they do not state at what current or view angle theses values are valid. The safe assumption is between 20 and 50 mA.
Converting candela to lumens requires the apex (view) angle of the spectral radiation distribution.
I assumed its sri to be 180 degrees
High and Mid Power Lighting LEDs are typically rated at a view angle of 120°. The RGB LEDs used in this strip are likely indicator LEDs where the angle is much less like maybe 10° (like those shown below).
180° may be the correct apex angle, maybe not.
457.5 × (2π(1 - cos(120°/2))) = 1,437 lm
457.5 × (2π(1 - cos(12°/2))) = 15.7 lm
LINK: cd to lm calculator
You datasheet is only for the LEDs and not how the manufacturer of the tape used them (e.g. LED resistor values).
Using the tape wattage use a loss of 46% for the resistors. The blue and green lose 40% to their resistors and the red 60%.
At 20 mA the LEDs would use about 14 watts, and the resistors 6.44 watts or 20 watts. So if your strip is 45 Watts the LEDs are drawing about 45 mA. If this is true the resistors would be about 45Ω.
The roundabout average output of a 10W white LED bulb is 900lm
This presents another problem. Incandescent light bulbs are isotropic (360°) and LEDs are directional. Usually 30° is subtracted for the base obstruction. Even though LED bulbs are not isotropic their luminosity is measured the same.
There is NO WAY you can compare the efficacy of these RGB LEDs to white LEDs used in a quality light bulb.
You have insufficient data to guesstimate the lumens.
Even if you had all the data, you could still only guesstimate.
You would also need the transmission loss in the light bulb's diffuser cover.
The best way to determine the luminosity of the tape is to light up a dark room with the tape then light up the room with a light bulbs and compare perceived brightness.
Everybody knows that the white LED cobs in bulbs are far more
efficient than incandescent bulbs
Most light bulbs use mid-power LEDs because they have been more efficient than CoBs. The highest efficacy LED currently available is the Samsung LM301B @ 229 lm/W. OSRAM just announced, in the past month, the highest efficacy CoB @ 201 lm/W. Currently available highest is a Samsung COB-D Gen3 @ 182 lm/W.
I removed the disffuser covers from some light bulbs I use around my house.
Notice the heatsink the CoB bulb uses.