Reddit mentions of Intros, Endings & Turnarounds for Keyboard: Essential Phrases for Swing, Latin, Jazz Waltz, and Blues Styles

I don't have a teacher but end up practicing at least a couple hours everyday. It's all about your own diligence in challenging yourself to learn things that are unfamiliar. You only get good at what you practice. A good teacher is able to hold you accountable and provide new things to learn, but I think you can accomplish that on your own too.

My routine consists of:

Hanon scale exercises. They're written in C, but I cycle them through all the keys.

I got a book of popular chord progressions and I work through at least one a day, transposed in all keys.

I bought a hymnal and play through 10 a day, each only twice. The songs are simple and numerous and has greatly improved my sight reading.

I keep a some classical songs to work on. Currently Chopin and Satie.

I compose. Not a whole lot, but a little everyday adds up. I think composing your own song gives you an intimate familiarity with how it is constructed and allows for greater ease of variation. Compose something you can't master in a day, something difficult that will expand your ability as a player, both technically and compositionally. Don't be afraid to create something, learn it well, and then forget it. It's more important to focus on the process than the product.

Find somewhere to perform. Nothing spurs you on to practice more than an audience.

Figured bass is just too limited for this honestly.

You're closer with the C9/D idea. The best solution I've found for transposable chord notation is the one used by John Valerio in this book and it's really changed the way I sketch harmonic ideas for myself. Unfortunately I don't tend to use it broadly online because most people aren't familiar with it.

Basically he uses Roman numerals, but like many from a jazz standpoint he doesn't use the upper or lower case to imply major or minor. Instead he only uses upper cases. He then specifies the chord type so say Dm7 in the key of C would be IIm7 (rather than ii7). Something like the traditional V7/V (D7 in the key of C) he just writes as II7.

This means he can write inversion with slash notation combined with Roman numerals. C/E would be I/III. Your example of C9/D I'm assuming is in the key of F (because it's dominant). So it would be V9/VI.

This also makes things like tritone subs and other non-diatonic ideas easy to express. Something like IIm7-bII7-IMaj7.

Also, short jots into other key centers are pretty easily expressed.

At first I thought this was insane, but after spending some time with it I found I adapted amazingly quickly and found transposition easier. It's like the Roman numeral equivalent of moveable-do solfege. I very quickly got good at thinking of secondary functions in particular as their alterations with respect to my primary key. I may have had a slightly easier time adapting because I already think of diatonic relationships within a given key as a necessity of chord building in jazz. If you're already good at this, then it's pretty trivial.