I haven’t seen much variation in resistance vs listed weight on any of the machines i’ve used, and i’ve probably been to ~30+ gyms over the last 10 years. You’re likely at that true weight, but the weight you’re doing isn’t unreasonably heavy - you’re just ready to increase weight/reduce reps in my opinion.
Wasn’t aware of this until I was informed by @mnben87 - I believe you should continue with this weight, or do slow progression with it until you are comfortable with this movement. DONT fuck yourself up chasing weight here, you’re doing fine.
As for plank, nothing wrong with doing it for 20s but if you are - you should try it in a much diferent light. Try this to stick with 20s planks…
“A plank is a perfect example of total body tension and co-contractive forces. It’s a battle of near-maximal contractile strength from all areas (shoulders, abs, glutes, low back and quads) to produce a steel beam in human form, able to withstand any external forces.” - Dean Somerset
TL;DR: Do a regular plank, then contract literally every muscle in your body as hard as you can and hold it for the prescribed duration.
Nothing wrong with sticking to a program, I do believe you need some minor adjustments but overall i think you’re doing just fine. Keep it up