Generally speaking, as you have rightly observed, you need to have an appropriate rubric serving as your analytic rating guide because you want to assess learners' speaking performance for making decisions about their rate of progress.Here, you need to specify certain criteria and standards showing varying degrees of success in learners' performance by writing clear qualitative themes. Naturally, these qualitative statements guide you to rate the participants consistently. As an illustration, such statements should be based on predetermined behavioral objectives such as rate of delivery, language use, topic development, etc. As for your second question (i.e. triangulation), in qualitative research, it is common to measure a construct using different techniques. Therefore, after rating the learners' performance, it will be a good idea to recheck them by an other instrument like interview, role play or discourse completion test (DCT).
Either by recording student's speech then analyze it according to certain criteria in relation to the aims of the study or by making oral interview with them one by one.
Your observations on student' feedback is rightly pointed through your question. Generally as the mentors, We do conduct debates. Group Discussions, Case studies, Just a minute sessions to assess their speaking ability. In case of individual assessment, we can focus more on one to one interaction in the form of interview or a role play.