As someone who has recently tried to refactor our app atop of VSCode (treating it like a platform), we got burned by the UI design decisions that are not straightforward to overcome, let alone maintain. The closed-source MS marketplace did not help either towards our OSS goals.
However, I found Theia (https://theia-ide.org) on HN (like a bunch of other cool things; this is one way I justify the time I spend/sink on this site) and find it a much better fit for our OSS goals (foundation owned, open-source marketplace) with full mod-ability while being compatible to VSCode extensions API (in theory). I recommend you look into it for your app.
However, I found Theia (https://theia-ide.org) on HN (like a bunch of other cool things; this is one way I justify the time I spend/sink on this site) and find it a much better fit for our OSS goals (foundation owned, open-source marketplace) with full mod-ability while being compatible to VSCode extensions API (in theory). I recommend you look into it for your app.