Plank on your elbows? That shouldn’t really require any arm strength. Are you using a push up position by mistake?
Either way, the only way to really reduce the strain is to either set up a resistance band under a pull up bar to take some load off, or get someone to pull you up with a towel under your mid.
If you are not strong enough I would say that just carry on trying. As you lose weight, that 1-2 seconds will slowly grow in duration.