What is the Document ID?

We do not use this or parse accounts documents so I know little about this. Fortunately it’s possible to search this forum! You can also, but when I did I see some of those questions are maybe answered here. In fact it looks like a thread asking some of these questions (with answers) has come back to life:

https://forum.aws.chdev.org/t/getting-financial-data/4605

There were some other enquiries about the individual points you made:

Employees:

https://forum.aws.chdev.org/t/can-i-get-number-of-employees-from-api/908

https://forum.aws.chdev.org/t/number-of-employees/4755

Capital
Probably you will need to look in any accounts filings.
Companies can file a statement of capital but not sure if / when that is required.
Previously this could be reported as an associated filing with the Annual Return but that type of filing has not been made for a number of years. (Example here in one example in my post):

https://forum.aws.chdev.org/t/document-description-values-for-filings-api-call/4256/2

Reserves
Again you will probably need to parse the Accounts filings.

You can also find Accounts information in bulk. @ebrian101 has a guide on the bulk data here: Bulk Data products from Companies House | CH Guide

If you are planning to look at the Filing History list to find accounts filings in XBRL documents note that not all accounts are filed electronically / not all will have XBRL documents (some will be just PDFs e.g. pictures).

There is information on how to find out if there is XBRL data available for a filing and if so download that rather than e.g. a PDF in my post on the Document API I think.