Datum planes are usually unnecessary; cluttering up a design with a plane per sketch is certainly the wrong way to go about it.
You can place each sketch with the same tools you place a datum plane -- by editing the attachment and attachment position. (You just can't do it at the point of creating the sketch in the same way as you can when creating a plane).
You can then use expressions in those positions, including e.g. the width/depth/length of other objects (using the length of another Pad would be one very common scenario). Or you can use values from a Spreadsheet.
And TNP won't always bite you anyway. It's possibly better to learn how to fix attachment issues than clutter up your design with datum planes.
Learning how to place sketches in arbitrary space is so core to FreeCAD that it's probably worth going through the pain on this.
Datum planes do have uses -- like when you want to attach large numbers of sketches to a single plane, or when you want to model some arbitrary plane without using physical geometry. They are also useful for e.g. cutting holes up to a face (because the plane can substitute for a face)
They are also helpful, I find, when I am using an LCS for some feature.
But quite often you see people trying to work around TNP with a datum plane, only to attach that plane to the same object that will have the TNP.
This tutorial on attachment may help you (it's a bit involved)
Datum planes are usually unnecessary; cluttering up a design with a plane per sketch is certainly the wrong way to go about it.
You can place each sketch with the same tools you place a datum plane -- by editing the attachment and attachment position. (You just can't do it at the point of creating the sketch in the same way as you can when creating a plane).
You can then use expressions in those positions, including e.g. the width/depth/length of other objects (using the length of another Pad would be one very common scenario). Or you can use values from a Spreadsheet.
And TNP won't always bite you anyway. It's possibly better to learn how to fix attachment issues than clutter up your design with datum planes.
Learning how to place sketches in arbitrary space is so core to FreeCAD that it's probably worth going through the pain on this.
Datum planes do have uses -- like when you want to attach large numbers of sketches to a single plane, or when you want to model some arbitrary plane without using physical geometry. They are also useful for e.g. cutting holes up to a face (because the plane can substitute for a face)
They are also helpful, I find, when I am using an LCS for some feature.
But quite often you see people trying to work around TNP with a datum plane, only to attach that plane to the same object that will have the TNP.
This tutorial on attachment may help you (it's a bit involved)
https://wiki.freecad.org/Basic_Attachment_Tutorial
But the various Mango Jelly videos about sketch attachments are well worth watching.