First off, the DT + DN combination is a winner on many levels, including size and immediacy. And if you want an FM synth, I doubt there is much better out there right now than the DN. So my general thought on this is “of course get a DN!”
That said, if you are really looking for subtractive synthesis, you may wanna start with a synth that’s focussed on that.
I’m not sure what sort of sounds you are after, but I can at least assume you want something polyphonic given you’re eyeing the DN. Next question would be “how many voices?” And “does it have to have an integrated sequencer or would you be happy to use the DT’s 8 midi tracks to sequence?”
If you want to stay within the Elektron Universe (capitalised, baaam! ;)) I’d highly recommend the Analog 4 (Mk1 or Mk2 though I personally prefer the form factor of the Mk1). It’s a super capable and flexible subtractive synthesizer (that can also do a bit of FM) and it can be used in a 4-voice polyphonic mode. Awesome for leads, pads and (with a bit of practice) bass. It’s sequencer is comparable to the DT (but has some additions that make it a little more powerful, like pattern change modes & song mode). Price wise, depending on where you live, you could pick up a Mk1 used at a price equal to or lower than the DN (I paid £450 for mine)…which is INCREDIBLE value for what you get.
If you are not hung up on Elektron or having an Elektron-style sequencer built in, the options open up and it becomes a matter budget.
At the more expensive end, a Prophet 6 or OB-6 are amazing analogue polysynths with unique sound and joy for years.
At a mid-range-y price end for polysynths, the Novation Peak seems to be a crowd favourite on this forum. It’s a very nice synth for sure.
Similarly priced is a Dreadbox Abyss. It has (only) four voices but those are delicious voices (and it’s completely analog)
Going for the cheaper end, you’re looking at something like the Behringer DeepMind 12, which is apparently great value for money (depending on who you ask). I struggle to recommend Behringer at any point, but the DeepMind I think is worth a look.
Something like the Korg Monologue also is an option. Also limited to four voices like the Abyss, I believe, but also on the cheaper end (like the DeepMind).