I try to back edit beyond 50*200 pages.
Page 51 gives Too many results. Try using smaller searches or the id_above parameter.
https://www.inaturalist.org/observations/efmer?filter_spam=false&page=51&preferences%5Bper_page%5D=200&user_id=750774&utf8=✓&viewer=750774
Just fails, page 50 works fine.
add &id_above=[highest observation id returned] or &id_below=[lowest observation id returned] to your URL to control which observations get pulled back. so for example, instead of using your 51st page query above, run the query for the 50th page, find the id of the last record on that page, then run the query again, resetting the page=1 and adding &id_below=[id of last record on 50th page].