Co-founder Dispute in Startup- Next Steps?

I find myself in a challenging situation with a startup where I've been a co-founder for the past 8 years, and I'm seeking some guidance on how to navigate this complex scenario. Background: Eight years ago, I joined a startup as the second employee, and there was a verbal agreement that I would become a co-founder. This agreement was formalized later, but unfortunately, there is no written co-founder agreement. Currently, the founder and I are the only two directors, holding approximately 68% and 28% shareholding, respectively. Other shares are held by external investors. Current Situation: The relationship with the founder has deteriorated to the point where they are asking me to leave. While I acknowledge that working together is no longer feasible, I have reservations about leaving without a fair and amicable exit agreement. I have been a crucial part of building the company from the ground up and played a significant role in its growth. Key Questions: Termination Authority: Can the founder (68%) unilaterally terminate my position as a director, especially considering the initial offer letter that mentioned "at-will" employment before becoming a director? Exit Terms: I am expecting a gardening leave of 18 months and an early clearance of dues owed to me by the company (payable over 4 years). What is a reasonable expectation for gardening leave, and how can I ensure a fair resolution? Initiating Exit Negotiations: The founder has not initiated any action for a settlement, and I've also refrained from starting the conversation about an exit agreement. Should I wait for the founder to take the lead, or is it advisable to involve a third party to facilitate the discussion? Leverage in Negotiations: What leverage do I have in negotiations, considering my significant contribution to the company's growth and the absence of a formalized co-founder agreement? Note: I've not been to office since start of November and have done few tasks from home since then. Me leaving the company has been a sudden development, and I am not sure what job/employment I'll take up in the future. The company's sales are strong, but we have been facing challenges with inventory and raising finance, both of which fall under the responsibility of the founder(68%). These issues have added complexity to the current situation.