Is there a website that can intelligently help me map out a good path to unlocking higher-tier devotions? Like being shown the paths and steps required to spend/refund/re-spend the points to get the constellations I'm after?

Check out https://www.grimtools.com/calc/ . Not only you can plan your build, you can also save it and look at other players' builds.

Not really. You just have to learn how that can be done. You can use the grimtools calc Grotok linked to play around with devotions to work out how to get what you want, but it won't show you paths or when you can refund a constellation.

There is the odd play by play on youtube showing how to use devoctions. They often happen by temporarily removing the devotions from a "named" that does not give out rewards for completion

Thank you for the help. I have been camping out on the Devotions page and just accepting that I need to work with it until it starts to click. Seems to be the only answer!

Best way is to decid which of the tier 3 constellations you want (ones mostly around the outside), check their affinity requirements and then look at the other constellations to work out how to get there.

This might help:

https://forums.crateentertainment.com/t/devotions-a-how-to-on-maximizing-them/49384

That's part of the fun. Setting a goal for the higher tier constellations and mapping a way there as you see fit. Try out new ones, respec, make it more efficient, etc.