Ask IH: How would you value a codebase and resolve IP "disputes"?

I've worked on a project for two years with two others co-founders (one technical and one not). At the end of last year, the project was abandoned.

Since I don't want to just throw it all away, I've asked my ex co-founders if they agreed to let me reuse the code. One of them immediately refused, and proposed two options: (1) pay a flat fee to buy their part of the IP or (2), pay royalties if I managed to get money out of it.

I'm wondering about how to approach this in order to determine a value that actually makes sense and that isn't unreasonable.

For background details, check out my initial post: https://www.indiehackers.com/post/it-took-me-two-years-to-realize-that-my-co-founder-is-toxic-for-me-395b9a5d01

As well as the recent update that I've shared: https://www.indiehackers.com/post/it-took-me-two-years-to-realize-that-my-co-founder-is-toxic-for-me-395b9a5d01?commentId=-MtRUMCpddxrZsJj_yyX

The non-technical co-founder only participated in brainstormings, ideation, and weekly meetings, but did not contribute to the code otherwise. That person also paid 1/3 of the UX/UI design that we've bought (~3K in total).

Regarding the code, I probably own ~70% (if not more) of the overall code, as I've re-architected and built a lot of the project and had to rewrite many parts to fix architectural issues, bugs, implement the design, the build, the infrastructure automation code, the CI/CD pipelines, the release management scripts, etc.

Here's what I have in mind:
(1) Repay the cost of the UX/UI design (same price we initially paid)
(2) Evaluate a price for the codebase, and split it according to who owns which percentage of the codebase
(3) Take into account the current situation
(4) Take the known problems and bugs into account as well
(5) Ignore the time/effort spent working on the project, because otherwise the sum would de-facto be over 500K

Code ownership:
A basic check with a tool that generate git repository stats tell me that i am reponsible for: 72% of insertions, 83% of deletions, 70% of the files and 65% of commits (whatever that means)

The current situation:

• No founders agreement, no legal entity, no nothing
• 0 customers, 0 contracts
• No other interested parties
• Functional but incomplete project
• Codebase unmaintained for ~6 months
• Low overall security posture

Known problems of the project:

• Major architectural issue due to the choice of a NoSQL data store. The entire data layer needs to be reimplemented due to the performance problems, and the impact on the codebase will be substantial
• Low code coverage
• ~15-20 known bugs (low-medium severity)

If you read the posts, you'll see that I've been through quite a lot, so honestly I'm not ready for a battle with lawyers and such (and can't actually afford that either). I don't have much budget, so I fear that my ex co-founder will ask for too much...

Have you faced a similar situation already? If so, what did you do / what would you recommend?