I know this is coming in late, but I do have a recipe to share that I tend to use for muffins. It's from the Bravetart cookbook (yeah, I'm the one who talks about Stella Parks all the time, but I'm a creature of habit) and there are two major steps. One, you can make the muffin mix ahead of time and leave it on your shelf until you want to use it. It also provides a bunch of different options for flour and flavours, so kudos to that. I use other muffin recipes online, but I really like the options for this one. Two, you actually add in the liquids in order to make the muffins.
A few modifications I've done with the recipe (I've done a lot at this point, so bear with me). There's a lot of sugar and oil/butter in these. The recipe calls for 3/4 cup of sugar, but I've gone as low as 1/4 cup (sorry, in this case, I didn't weigh ingredients). If you go that low, I recommend adding in some fruit because it might be a touch savoury. Otherwise, I think 1/3 cup is a fair balance.
For butter/oil, I'm not as adverse, but I've certainly used 4 oz in the past.
With these changes, the muffins do not necessarily rise as much, but they still taste nice.
For the squash version, I've used spaghetti squash and used milk anyway and it turned out fine. In that case, it didn't give much taste, but if you have squash to use up, then by all means, put in what you want.
Whenever I've been out of eggs, I tried that baking powder/water/oil replacement. It works in terms of rise, but the muffin is more crumbly.
Finally, my phone camera sucks balls, so if anything is unclear, let me know and I can provide information.