Difference between 到, 到了, and了 to indicate completed action?
想 is the verb

到 is the result complement in 想到 ('think and successfully recall'/ 'think and successfully come up with')

想到 does not indicate what tense the action is in. E.g. 將會想到 (future tense); 此时想到(present tense); 曾想到 (past perfect tense)

了 indicates 'completed' action (but not an indicator of past tense)



想到 indicates a successful action (you think and come up with something)
想了 indicates a completed action (you thought but might not had came up with anything)
想到了 indicates an successful and completed action (you thought and recalled or  came up with something)