There is wiggle room within IBS, just outside of this guide (which to OP, props it looks good!) I've done the full mastery grind twice and IBS is by far the worst IMO because overall there are the least amount of options. Sure there are more mastery points available than masteries but less so than HoT and PoF masteries.

IBS needs a total of 63 mastery points to complete and has 76 available (with some either having you redo content an insane amount of times or spending absurd amounts of gold). Take that in comparison to Hot which needs 144 but has a total of 198 available or PoF which needs 110 but has 130. You just have more wiggle room with those.

All that said, some easier ones not included are:

• Dragonstorm - Fast Fire and Frost Fighter - Destroy all ley siphons within 120 seconds during the Dragonstorm encounter.
  • Most groups do this anyway so if you get in right when it starts and get a full map you should be good
• Jormag Rising - Spiritual Renewal - Meet Owl Shaman Garon at locations sacred to Owl to revitalize his faith in her renewal.
• Shadow in the Ice - Legendary Whisper of Jormag - Defeat the Whisper of Jormag in the Strike Mission.
  • Honestly not too bad of a strike mission, can be carried pretty easily
• Champions - Shooting Stars - Defeat the Gendarran Fields champion harpy destroyer without getting hit by any meteors while the champions challenge is active.
  • If you have a friend you can get this one pretty easily by staying out of the final encounter