You didn't say how you are developing your negatives. There are many formulas that will do better at holding highlight detail than D-23, which, even though it's a fairly gentle developer, because of its high sodium sulfite content is prone to highlight runaway.(posted 8217 days ago)A tanning formula based on catechol is probably your best bet. The tanning/staining property of catechol will help you retain highlight detail better than anything I've found.
The other critical point is, no matter what developer you use, you need minimal agitation and lower developer concentration. Many formulas are prone to streaking when used with minimal agitation, however. Those based on glycin and catechol are particularly good. Also, HC-110 and TMAX-RS do well, too. I suggest you dilute your developer by at least 100%. In conjunction, reduce agitation to 10- 15 sec. every 2 to 3 minutes, and extend your development times by 100%. You will not lose much film speed (1/2 stop, if that), but you will easily achieve N-3, with very good midrange separation. If you want even more compensation, dilute it further and after the first few agitation cycles, reduce agitation to every 4-5 minutes. I've achieved N-6 using this method, but I've had to extend development from my normal time of 10 minutes to 45 minutes. Be sure your film stays fully submerged in the developer, otherwise you'll get arial oxidation, which will really disappoint you.