Sleep training/ naps

So we’ve been successfully sleep training for almost a week. My daughter is 5.5 mos will be 6 mos May 1. We’ve been doing the Ferber method, and she no longer cries/fusses for more then 30 min for naps and bedtime. However we keep running into a problem that I’m unsure how best to fix. The last two days, her second nap has ended super early. Today, it ended at 2:20. Her wake window has lengthened to 3 hours. That would put her next nap at like 5:20. Bedtime is usually around 8. She obviously won’t last til 8 tonight if she doesn’t take another nap. I’m not sure if I should try a cat nap in a little while? Would it ruin our progress sleep training if I let her take a cat nap while nursing? Even just for like 20 min to help her get to bedtime? I know if I put her in her crib, she won’t go to sleep right away, causing the nap to creep closer to bedtime. Last night we had the same issue. However she napped til 3:30 we kept her up til 7:30- she usually goes about 4 hrs before bedtime. She fell asleep on her own in her crib a little before 8 and slept til 5:50. I’m just not sure what to do!

Also she typically naps for at least an hour. Her last nap was 1hr 40 min. Her total daytime sleep today was 2hr 50min.