I have been there, too.
The price of a better real ‘WYSIWYG’ MD-editor for DTTG (and a little better one for DT), would be to ‘strike two flies in one go’, have a real inroad for more people to use DT as one-stop note-taker; and bucket the ‘read-it later’ bonus ‘on the same run’.
There is so much convenience and sense in avoiding the need for a parallel ‘reader’-system w/ highlights, which one later has to meticulously fuse back into DT somehow. all sorts of practical and cognitive frictions.
and, it really is about a more inclusive ‘save – read – highlight’ scenario, acknowledging all 3 parts are equally and simultaneously impotrtant – just as you describe. so no wonder, you find a lot of requests around the particular issue, and you maybe saw all the others (after doing a simple “read it later” search here).
so, here one take and a possible route to consider, resp. m2c:
for some people it works w/ going MD, and then plugging in external editors. but there is friction, not only for the reading, but also in most scenarios btw. mobile and desktop (often there will be different external editors involved).
for me, the issue was semi-solved by a better way to read and highlight with the (semi-)WYSIWYG-MD-editor on DT4.
now, the missing part is indeed the DTTG one… (and even with the new WYSIWYG experience in DT one has to settle with no way to get rid of / tone down markup codes or differentiate btw. headings and text in terms of further formatting.)
but, if you want to go the MD route, which really brings one more than close to an Instapaper reading experience IMO(!), I think there is one route to get decently close:
use a selected external MD editor which is good for round-tripping and reduces the friction of changing apps to the max; i.e. allows a simple, ‘two click’ path via sharesheet.
– for me that is Ulysses, as one can open DT-files (after downloading ‘clean’ versions to DT flawlessly) in Ulysses, and if you set it up for ‘external folders’ it will directly handle and save every edit in DT. Ulysses allows, unlike DT, for a CSS-set-u of the editor, which can not only bring you very very close to a WYSIWYG-experience, but also to one of your liking.
As anyone coming from the literary world is aware, for a lot of people reading is a very intimate thing and an aesthetic experience alongside a cognitive one. so, this is worth gold. plus one can decently well handle highlighting in the same editor that also allows some pleasant reading experience.
(I one did something likewise with Obisdian once, just to mention that as another possibility to explore; but that was when Omnivore was still around, and that directly plugged into Obsidian, and that could – in turn – be plugged into DT via indexing…; but I am sure there are other plugins allowing RIL via Obsidian, and syncing this with DT)
Maybe that´s a way, until DT hopefully also allows for better MD-editor experience in DTTG (which seems possible, even if I never read that this is on the mind of the dev-team yet.)
So, hopefully threads like yours give further encouragement here. 