Assume we have 2 accounts:
1: A bank account for payment of the share purchases when they are due.
2: a share account for holding the shares.
The initial transaction is to purchase the shares for full price into the share account and select pay for the shares from the share account balance. This will record the purchase of the shares for their full value, reduce the cash balance in the share account by the cost of the shares and increase the share account value by the value of the shares. There is no net change in the value of the share account.
As the shares change value (price fluctuation), the value of the share account varies, indicating the share value is fully covered by the share account valuation, ie there is full risk exposure to the share purchase.
As instalments are due, the cash payment is transferred from the bank account to the share account, increasing the net value of the share account and reducing the bank account.
When all instalments are paid, there is a net zero cash balance in the share account and the full value of the shares. Compared to before buying the shares, the share account has now increased by the (current) value of the shares and the banking account has reduced by the purchase price cost of the shares.
This is assuming the shares may be listed and their price may be varying over the time instalments are paid.
My preference for this approach is that if the share price varies over the period of instalment payments, the net worth of all accounts reports the full gain/loss associated with the share ownership.
During this period, when recording the share price, you have to record the fully paid share price, although the market reported share price might be adjusted, based on the number of part payments. However, this would not report the full risk exposure to the shares, should they vary due to market conditions. It all depends if the shares can be traded as partially paid or fully paid, but I suspect this can vary. There was a Qld motorway share that had some strange instalment costs that cost some punters a lot of money due to this structure, as they were traded as partially paid, while the instalment outstanding was substantially more than the sale price.