Not sure if it helps, but I kind of do this by using a combination of Custom category and tags. I always want to see how much a trip cost in total, so for every trip I create a new tag. I categorize all my expenses with the normal categories (e.g. food, drinks, transport) and then add the trip tag.
After the trip, I just search for all transactions with a tag and am able to drill down on each category if I want to 
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Yes, that’s also my thinking; tags can be used as sub-categories very easily.
We are still struggling to see a use case where it’s better to have 2 custom categories on the same line vs 1 at the top and 2 inside (sub).
My post above is a really obvious use case - I can elaborate:
For me I want to just budget high level so use top level caregories as:
Income
Bills/Subs
Everyday Spend
Luxuries/Ad Hoc
Exclude
Sub categories would mean you can then break down into useful analytics like the traditional categories.
It could then look as follows:
Income:
Salary
Interest
Refunds
Bills/Subs
Utilities
Mortgage
TV/Music Subs
Loans
Everyday Spend
Eating Out
Groceries
Transport
Alcohol
Luxuries/Ad Hoc
Holidays
Furniture
Electrical
Exclude
Transfers
Credit Card Payments
I have a similar but different split id like being
Holidays
- transport
- eating out
- Accommodation
- Flights
- Activities
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