Back early last decade, I used a UNO with a ZIF shield and created a little ISP programmer for Atmeg386P-PU and Attiny85 DIPs. Bootloader at the time was Optiboot in an early stage:
/* Copyright 2013-2015 by Bill Westfield. */
/* Copyright 2010 by Peter Knight.
Anyway, my personal stance on bootloaders are they are generally a Pain In The Chip ...
I use ISP programmer:
The scripts documented at that time (Aug 2013) for Win 8.0/8.1 did work with confirmation that they also worked on Windows 7.
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Ray