Personally I think you may be over-engineering this as based on your question it seems you are concerned about indicating a data-point is clickable and user not knowing the information they will receive when clicking on that data-point.
Short answer for this question is style them as links (as essentially that's what they are). Based on the premise of the concerns you raised in your question, I'll break my answer down into two parts.
Indicating a data-point is clickable
Now I know you stated that "readability of the data suffers with this approach", but this is not the case. Firstly I'll refer to the W3C Technical documentation to highlight the accessibility importance in styling links. In Guideline 1.3 - Provide highlighting for selection, keyboard focus, enabled elements, visited links the guideline states that:
The user can visually distinguish between selected, focused, and enabled items; and recently visited links (1.3.1); with a choice of highlighting options that at least include foreground and background colors, and border color and thickness (1.3.2).
In particular, I'll reference the intent of guideline 1.3.1:
1.3.1 - Users need to be able to easily discover web content they can interact with. One effective way to do this is to highlight enabled elements and links (including recently visited links). Highlighted selection and content focus lets people who use keyboard, gesture and speech input know where they are working. On some pages controls may be difficult to discern amid a large amount of other content, or may be styled so the controls are difficult to distinguish from other content. This can be particularly difficult for people with visual impairments, who may not be able to distinguish subtle visual differences. People with some cognitive impairments may have difficulty distinguishing between items with similar or non-standard appearance. Visually distinguishing these items reduces the amount of time or number of commands these groups require to examine a page.
By styling these data-points as links, it will actually help visually impaired users (and users who aren't visually impaired) identify what is available to be selected, what they are currently focusing on and what they have already visited.
Taking advantage of a well established convention
The styling of a link is a well established convention in which there are perceived affordances already associated with it. Taking advantage of this established convention, the majority of users will quickly understand that the data-point is clickable. This Nielsen Norman Group article discusses about that you can style the link to go beyond the blue text / underlined standard, but stresses the importance "signalling clickability" (to ensure that any signifiers aren't lost).
A user not knowing the information they will receive
I think that you can leverage a combination of the perceived affordances of links in tables and the labelling you have used in the table for a user to understand the information they will be receiving.
Firstly your rows are clearly labelled as to what each line item is in the financial statement, which indicates that the value is associated with that row. Secondly your columns are also clearly labelled to indicate what time of year the value is associated with. This should provide an initial indication to the user as to what the value is about.
Secondly to talk about the perceived affordances associated with values styled as links in a table. As styled links indicate to the user that there is information to be accessed by clicking the link, styled links in a table indicate to a user that there is a function that can 'drill-down' on the associated value. It's seen commonly in major products such as SAP, Oracle and Tableau.
In summary, though styled links may not the most exciting answer for this question, it is the best way to indicate to the user that a data-point in the table is clickable.