Hi. By partial I mean only do one or two ties to lengthen the frenulum, but not so that it is down to the penis head (so not like @gizmonkey's).
I'm concerned that if I only do one or two shallow ties to lengthen by say 30% that this will weaken the remaining intact frenulum making it more susceptible to tearing in the future
I don't think it will be easier to tear after tying. Any tie will widen the area of attachment, spreading out the tension. That's been my experience, and I've never heard of a guy's frenulum tearing after tying. Before, yes, but not after.