First, good on ya for being curious and taking the time to do this.
Second, yes, all of the above. Part of the test method (chin ups) is unusual, and for untrained people, lactate will climb quickly.
Your warm-up and stage length are good. No need to change anything there. However, lactate values are sport-specific. (The same athlete will get different results for running, biking, hiking uphill, etc.) The more the sport movement patterns have in common, the more similar the values will be, but combining knee bends and chin-ups could lead to confusing results.
Can you do the test again, but in constant, uniform motion between samples? If you’re a runner, do it running. If an alpine climber, do it hiking uphill.
If you have a friend with a treadmill, that will be the most convenient and repeatable way to test. (Use it on an incline if necessary.) However, a treadmill in a public gym is probably a no-go; they’ll object to taking blood samples on their machines.
If you do another test, once you get to that ~1.3 mark, start testing in 5-beat increments rather than ten. That way you can zero in on the ~2 mM mark. No need to go finer than 5-beat increments; it’s probably impossible to do accurately anyway. For example, in your first test, it would be good to know what lactate is at ~95 bpm.
Once you have the data from your second test, you can plot it on a graph for future comparisons.
If you get similar results to the first test, then yes, you’d be best served by staying under 100 bpm for now, probably even 95 bpm.
Lastly, do you know your max heart rate? If not, what’s the highest heart rate you’ve previously recorded? In what context was it recorded (how hard for how long)?