Methods to de-harsh old Reason instruments?

[donkey tugger]

Bounce the instrument tracks to audio at highest sample rate possible and then use TDR Nova (free edition is sufficient) to tame the most prominent resonant peaks in the mids and highs using static and dynamically reacting narrow bell filters with fast attack.

Other options to try:

Toneboosters Sibilance v3 (set to freeware status recently) or the fully functional v4 demo

Vastaus by Wavegrove (says "high-end enhancer", but it's actually more of an de-harsher)

Tokyo Dawn OD DeEdger (has a fully functional - except saving plugin state - demo version that can be used to process and bounce your material)

In addition to these good options Limiter No6 also has an excellent HF limiter module which is marvellous for smoothing transients and HF harshness;

https://www.tokyodawn.net/vladg-limiter-n6/

[Terminal E]

Have you tried any of these EQ plugins like Oeksound Soothe or Soundtheory Gulfoss on your recordings or masters? Really good for getting rid of harshness