Calculating weighted average for stocks in Numbers

I am in need to help with a formula, I have completely gotten stuck and can't figure out if I am missing something obvious or its not possible at all.

I use Numbers to track the performance of my stocks and cryptos, with a few extra tools I find handy for decision making. Example of transactions relevant to current topic:

With the info from above, I use the following for dollar cost averaging. Here:

The idea is to have everything automated except the "enter" field. If I want to enter let's say 250 to AVAX, it should automatically give me the New Average and the percentage increase.

I got this working by manually entering in all the data fields:

Using the above I had no issues, but that would require me to adjust the formula after each entry.

To automate this I tried replacing the formula with:

Whatever I put into the Enter field doesn't work (using the above SUMPRODUCT formula).

In this case, the new average should be lower than the current average and the improvement should be positive.

Is there a mistake in my approach or another way to achieve my intended goal?

Thanks in advance,

Remy