The time has come to book some DONE4 or DGLOB34 tickets. We have completed a number before, but not for many years. we would appreciate any tips on the items below.

• We are planning to start in Oslo. Is there a local agent people used? Otherwise which airline would you recommend to book through? The first segment will be Qatar, Finnair or BA.
• We are looking at starting in Jan 2023. D class already seems sold out for LHR - MLE per Expert Flyer. I am presuming that is unlikely to change and we should therefore look for other routing.
• Are people generally finding it hard to get D class. For example, we will need SYD - SCL and it looks like very limited D class on Qantas for any moderately busy period

Thanks for any advice

There is no need to use a local agent anymore (this was required some years back). You can now book with any airline or agency you prefer. I know different airlines calculate the airline-imposed surcharges differently, but I don't know which airline might be cheapest. I'd guess that BA might be more expensive, with AA perhaps not as bad but still higher than others, but these are just guesses on my part. Many here use the AA RTW desk, others like BA or QF or CX, etc.

D inventory is sometimes hard to get. You can try using different points of sale (PoS) in EF to see if that makes a difference. If it does, that might lead you to use an airline or agency in that country, or call the reservations desk of AA or BA or another airline in that country. You can also try different codeshares, e.g., in EF set the "Alliance" control to "OneWorld".

If you're not starting with a Qatar Airways flight, then you can always try using the online booking tool
Good luck with that, though I did manage to use it back in March - see this post

The tool claims it can issue Global Explorer in addition to Oneworld Explorer tickets, but nobody has yet reported trying that

I expect January is the peak of the peak for people wanting to fly LHR-MLE, so BA might well restrict availability of discount business seats (which D booking class is)

If you want an out of the way tropical island, on a Global Explorer ticket Christmas Island is possible (but only one flight per week, so you'd be stuck there for a minimum of seven days)
(the Global Explorer ticket allows additional airlines, including Fiji Airways)

ETA:
re your SYD-SCL flight, which implies visiting South America
Are you aware that you can no longer include LATAM flights on a Global Explorer ticket? See this post

You might also look at Lord Howe Island off the coast of Australia, with economy-only service from SYD.

Also note for the time being originating in Hungary is a little cheaper than Norway, and Japan is cheaper yet.

If considering alternatives to MLE, perhaps consider SEZ? I've been a couple of times and enjoyed it.