Single Arm Plank > What's the best way to increase load to work twrds next Progression

Hey y'all,
I'm still recovering from shoulder surgery and can't use my right arm. Starting rehab these days and will slowly but surely work through all the upper body progressions of F1 once my shoulder is able to take some load.
Meanwhile I've started some single arm work with my good arm just to keep using it and because it's still a lot of fun. So now I'm on sPL/PE4 and step 9(5x30sec) is starting to get fairly easy...of course because I'm not doing the 2nd 30sec on my other arm and therefore challenging my core a lot less.
 Would you recommend a) to go for longer sets (e.g. up to 5x60) or b) go for more lean and slowly work twrds sPL/PE5 (as far as it's still manageable to counter the torque).
What's the elbow position with straight arm? Cubital fossa pointing inwards or to the front?
And last but not least any other single arm exercises I could throw in to mix things up while waiting to get back up to speed with both arms - which is after a few months of rehab? (right now doing single arm planks and incline rows)
Thanks! 
B

Hi Bas my personal advice it is that your approach will leads probably on imbalances , my preference it is to work with the healthy arm as minimum. I know that usually when we have an injury it can be fine to use the other limb and that its work can be useful on the affected arm. BUT since the plank exercises lead to strong scapulae work i do not want that you work only one side.

for the elbow position the ideal orientation should be with the fossa pointing forward.

@Alessandro Mainente thanks for the feedback. I was thinking about the issue of an imbalance, but considered the injured side to catch up once rom is ok and I can put load on it. I'm not maxing out on the good side. After 2 months lay off from any upper body exercise I'm dying to get back to it. I got your point though and will  just do a little maintenance work with my good arm once in a while. 
Thanks for pointing out the right fossa orientation, too.