Growing On Budget Brokerage

Hello

I recently moved my brokerage account on budget. I did this because there was a lot of money in there that was inaccessible by the budget but was money I realistically was going to spend, maybe on a down payment for a house or something. I work in finance so I understand and am comfortable with the risks of brokerage over a regular HYSA. My long term objective/goal here is to create the ability have access to investment fund growth without it being tied up in my retirement accounts for 30 years. I hope to retire a few years early so I want to grow this late career stage nest egg.

To compensate for the volatility in the account I created an "Investment Gains and Losses" account that holds about 35% of the accounts value from the initial transfer. Once a month I reconcile the account and any losses from the "Investment Gains and Losses" category. Any gains are usually split - 75% into Gains and Losses and 25% into random budget categories based on need.

My trouble is how do I add money to this account? Let's say I feel like I'm sitting on too much cash so I want to add another $6,000 into the brokerage. Well, I'll need to add between $2,100 to $3,000 to the "Investment Gains and Losses" budget line item, but every dollar is already assigned so where does this "extra" money come from. It's no longer just a transfer between accounts if I want to maintain at least a 35% buffer.

I also thought about creating a monthly budget line item for a transfer to brokerage (say $300/month). Every month I'll assign $300 to that category to be transferred to the brokerage, but I'll still need to come up with the additional funds to move into Gains and Losses. Would it be easier to open another brokerage, keep it off budget, and set up auto transfer?