Try Hagon. Not as posh as Maxton or Ohlins but probably better than the shock the DR came with as standard. I had a Hagon fitted to my NX650 and it was very good
I had a new YSS rear fitted to my 1991 DR 650 last year. Now I’m not a good enough rider to be able to tell the difference between shocks but it seems fine to me. I seem to remember it was just short of £400 so probably the same unit. I’ve done a few off road days on it and it does the job.
I wasn't sure about getting the shock rebuilt on my last XR250 (1991), spoke to a local motocross suspension specialist and they sorted it for £100 ish.
Should be do-able on the DR as the rebuild kit seems to be readily available online