I think I more or less on a similar journey but I've been through the 12-18 month cycle across a pretty wide range of topics (niche websites, affiliate marketing, writing on most blogging platforms, stock photography, ecommerce stores, dropshipping, courses, ubscription newsletters, Amazon KDP, etc.).
A few thoughts:
-I think 18 months is certainly a reasonable amount of time to asses Medium for upside potential. You are already doing well so don't be so hard on yourself. I think hitting $1,000+ per month is 100% possible given the trajectory described here and with a slight increase in article publishing).
-You really do have to celebrate the little wins. For me it's been over a decade of what at the time felt like losses. Unfortunately, this never really changes, in my experience, even when you do find some success with multiple income streams. So perserverance is key.
-Going all in is scary. But I always try to reframe my thinking so that I remeber the flip-side: the fear of someday looking back and wondering "what if" (there is risk in not pursuing your true passions).
Keep going! I'd keep Medium as an income stream, but it is very tough to stay on top even if you do make it to $3000+ per month range. Very few writers can keep this up for a variety of reasons. I know this from having interviewed so many Medium writers.
So I guess I'm saying going all in on something like Medium can be a good idea-to accelerate your long term earning potential through that channel (hopefully some period far less than 18 months). However, going all in for 18 months is a real risk. There is tremendous variation among top earning writers month to month. So much so that even if you did reach that level, you wouldn't want it to be your only source of income.