I'm planning to have some final conversations to try from the bottom of my heart to get my parents on board. Any replies would provide great insight inshallah.

Pray istikhara and talk to them asap! And get the potential to talk to your parents, I'm sure they'll find a common ground, and everyone will compromise to make every done in an Islamic way, inshallah.

Has anyone else on here had to seek a spouse without their parents, and how did you get them on board on the end?

You just can't. You can't represent yourself as your own wali. And there are certain conditions that must be met to change or elect your wali; the wali is the closest living person (your father comes first, then brother, then grandparents, and so on). You can elect your wali because they think you're not ready for marriage or because they don't approve of your potential... Moreover, it is not permissible for a man to marry a woman without the permission of her guardian (wali), whether she is a virgin or not, and this is the saying of the majority of scholars, including Al-Shafi’i, Malik and Ahmad and so on...

My heart goes out to the reverts that have to deal with this struggle at every major step in their lives, subhanallah.

I'm sorry, sister, but you have it harder than reverts. A revert can elect their wali, you can't. There are conditions for a guardian to be a wali for a Muslim sister, such as being a Muslim; if your guardian is not a Muslim, he can't be your wali, thus you can elect a wali. For the sisters in Western countries, there are many Islamic institutions that take care of such matters and will marry them.

I'm struggling without a consistent figure of authority who can give me direction and guide me in this process. I'm struggling without the "guardianship" of a mahram, of someone who's known me for a long time.

Pray istikhara and have tawakkul on Allah SWT. You'll be fine inshallah.

I'd really like to get my parents on board though! Has anyone else managed it?

I don't know anyone who have managed it, but I have a cousin who married a woman his parents didn't approve of, they haven't talked to him in 8 years and till this day they are still angry at him and make dua against him all the time.

The Islamic process of marriage is very simple, sister. Don't put yourself in a situation that will anger your parents till the day of deen and goes against the teachings of the prophet PBUH and the rules of Allah SWT. It's not worth it.

Just talk to your parents and see where compromises can be made on all ends, and make sure they talk to your potential inshallah. May Allah SWT ease everything for you. I say that and allahualam.