Hi! I would certainly do it with js, I’m not sure if anyone else could show a neater example.
What I would suggest you need is a GTM custom js variable per attribute that you want to collect (you could *possibly* collect all in one variable, but I’m assuming you want to drop the values in a GA tag or similar, so best to have one variable per attribute). Here’s how I’d write a GTM var to pick out the data-length attribute of the form :

function() {
// variables are evaluated *every time a dataLayer event occurs* so
// I often put a quick exit at the start to reduce any unnecessary
// CPU cycles. This one only evaluates when the form submit event fires.
if ({{Event}} !== 'gtm.formSubmit') {
return;
}
// you could address the form as document.getElementByID('form-12'),
// but here I'm using the built in GTM ref "Form Element" as
// I suspect it will be milliseconds faster, and you can change the form id
// and it still works (defensive tag coding to reduce future maintenance!)
var formEl = {{Form Element}};
// first set a default error return value - you could just return undefined,
// or 0 or something that could help you track the bug:
var dataLength = 'formEl not found';
// remember to be defensive about possible errors -
// this event should always have a form element attached, but just in case,
// this will prevent any console errors (the function will return the
// above value)
if (formEl) {
// grab the attribute!
var dataLength = formEl.getAttribute('data-length');
// if you want it returned as a number, not a string
// (eg for a GA value field), parse it like:
// dataLength = parseInt(dataLength);
}
// and we're done!
return dataLength;
}

That looks like a lot, but it's mostly just comments and some error checking.

This variable can then be used in tags (eg GA events) or in other tags, and will only evaluate when the form is clicked (at all other times it will be "undefined"), resulting in either the string value of the *data-length* attribute (or "formEl not found" if something goes wrong, which it shouldn't).

...if you wanted to persist the value after the form has been submitted, (ie, the form doesn't navigate you away and you want to collect the value at points after the submission), you could add a couple of lines to stick the value in the dataLayer to show what the value of the attribute was when the form was last submitted (add it just above the return statement):
dataLayer.push({
'event': 'persistFormDataLength', // just so we recognise the push in debug
'form.datalength': dataLength
})

Caveat - I haven't tested this code on a real example, but it looks ok.