Need formula advice for a personal portfolio management excel.

Im creating a Finance excel and fear ive bitten off more than i can chew. I have extensive experience with excel but could use advice in one area.

Background of Excel. The document has a 'transactions tab' that manually tracks all financial trades made, by Date, Account, Asset, Quantity (there are more columns not relevent for this). I have sheets for manually input price data for each asset, this culminates in a 'price master' tab containing Asset Date Price table, of all the price table data (note it doesnt contain dates of weekend trading for securities).

The goal was/is to create an "As at" portfolio valuation across all accounts. This sheet contains Date (sequentially down), Account 1, Account 2, Account 3......

I have spent a while trying to create a formula for this as at valuation. It is required to calculate the total account value in each account column as at the row date. To do this, it must first calculate the cumulative quantity of unique each asset (from the transactions table)(row date and all prior dates) multiplying it by that assets price for that date in the price table. The total of these will be the total account value as at that date. (when a date isnt available it should find the last date with a value and use this, this part i have solved). The workbook utilises some heavy formula but this one has stuck me.

I have tried creating a positions table, but had difficulty stopping the end of the sum if when the date of the transaction went past the date it should calculate to. Any help or advice is appreciated, happy to provide extensive additional detail and example formula.