I've been meditating for 4 months now. Started off doing 10 minutes. Eventually worked my way up to doing one hour per day, with 2 or three sessions of one hour each on weekends or whenever spare time was available. Got a taste of the 1st jhana maybe 5-6 times but am struggling to get back to it. As of recently i've been losing motivation to practice, and I've been considering switching from samatha-focused TMI style to mahasi noting (which i've been doing in daily life occasionally). 

So my question is: should i focus strictly on developing concentration to the point where 1st jhana is consistent (i.e. i can get to 1st jhana in 90%+ of sessions) or should I start incorporating some vipassana without worrying too much about jhana?

Vipassana without worrying so much about Jhana. Consider a Goenka course. It is an intensive course, free and will get you somewhere that is for sure. 

​​​​​​​https://www.dhamma.org/en-US/index

Try these hacks for jhana.
https://www.dharmaoverground.org/en/discussion/-/message_boards/view_message/14564976#_com_liferay_message_boards_web_portlet_MBPortlet_message_14599759
then at 2nd jhana switch to insight perhaps...test and see what works.

As far as goenka, great free retreat but ignores seeing hearing thinking in favor of feeling as definition of insight/Vipassana.
Good luck,
​​​​​​​~D