Keep bedtime same every night or adjust based on last nap?

As the title says - my baby is almost 6 months old and her sleep isn't great. I'm lucky if I get a four hour stretch, quite often she wakes every couple hours and refuses cot.

Currently I adjust her bedtime based on when her last nap ends, e.g. if her last nap ends earlier she gets an earlier bath and bedtime, although it's normally bathtime between 6:30 - 8pm latest. We try to keep a 7am wake up.

I've never managed to get into a schedule during the day because it's very random how long she will sleep and when for naps, although we are roughly following wake windows of 2.5 hours and 3 before bed.

I wanted to see if it's better to keep the same bedtime no matter what the naps are like, as I feel like it could be disorientating having a different bedtime every night? Or is it better to adjust based on last nap to ensure a big enough gap for sleep pressure?

Grateful for thoughts!