Google’s actual research does help people like me, by making our notes less awful digitally. But I’d love not to be dependent on tech innovations to make my handwriting better.
You just have to have the time and interest to do the work, much like you probably did when first learning to write.
I’ve done a few handwriting workbooks, mostly consisting of block letter templates to fill. But each of them is incomplete on its own, and they don’t even share the same font, making it much harder to practice.
My handwriting is immensely better with a fountain pen than with ballpoint or gel pens; I suppose partly because the fountain pen forces you to an optimal position and angle (it's much more inflexible about that, you can't just push it against the paper in any angle and expect it to write), and partly because it provides a smoother experience and feedback.
You don't need to go overboard, the typical €20-ish Pilot Metro with medium nib or similar is more than enough.
Deliberate practice (as in worksheets or exercises) is less important than just going as slow as you need to, to make the characters correctly, until your muscle memory builds up and brings back the speed.
https://handwritingrepair.info/
or see:
https://sites.google.com/view/briem/free-books
or John Howard Benson's lovely _The First Writing Book: Arrighi's Operina_ or Carolyn Knudsen's lovely _An Italic Calligraphy Handbook_ (which is much better than the modest title implies) and get a chisel edge marker or fountain pen.