I really want Musescore 3.0 to stop automatic placement

There must be at least 5 issue reports relating to fingerings. Perhaps these will fix all of your fingering issues. I know that fixing these is a priority for some programmers. If you move your mouse to activity on the menu and click issues, you can do a search for "Finger" and find all of the issues related to fingering. You can then click the 3 dots to the right and subscribe to them so you can see when they are updated.

I haven't done any purely choral scores, so I live with the non standard placement of dynamics and lyrics until I'm done entering a score. I then worry about layout all at once. I can select a staff (or all vocal staves since they're adjacent), right click a dynamic, select all similar elements in selection and set them all to above at once in the inspector. It is almost never necessary to move an element type one at a time. If you are patient, you can move as many as you want at a time. If you've tried it, you have seen that it takes a little time and I thought my system was going to crash the first time I did it.

There's a scroll bar at the bottom of the inspector that lets you look at the right (or left) hand side of the inspector without needing to widen it. This is useful if you want a more narrow inspector but still want to be able to see the right side of it.

You don't add text to frames using the palette - you right-click the frame. What you used "instrument change" for should have just been "part name" - it's not a change at all. And the other text could have just been "frame text". The only element you've added that really needs to be added from the palette is the tempo text, so that's the only one marking in the entire piece you should have needed to disable autoplace for.

You have a whole bunch of text elements you've disabled autoplace unnecessarily and indeed harmfully. Like, the words "verse" and "chorus" - if you want thsoe above the staff, don't disable autopalce and move them manually, just press "X" to flip them above the staff. Or use the Inspector, and if you want all text of that style to be above staff, click the "S" button (for "Set as style"). Similarly the text you moved to the left to avoid chord symbols - better to have just set the text style to be right aligned, and perhaps offset -1p from there. Actually, if you start from the Jazz Lead Sheet template, much of this is et up for you already. But from what you have, I'd suggest using "staff text" for the text below, "system text" for the text above, then you customize each as needed.

In another thread I commented on the use of stretch where system breaks are the much better way to do what you were doing.

Basically, everything you did manually here could have been much efficiently and effectively just by changing style settings, and then you'd only have to do it once - save the score as a template, and every other score you create from that template inherits those same style settings and will look perfect right out of the box, no manual adjustments of any kind needed again (well, that's a slight stretch, but only slight).

I've fixed up your score using all of these techniques. Use this as a template (save it to your Templates folder, select it from the template list next time you create a new score) and you should find you need almost no manual adjustments, the few you do need won't require disabling autoplace. The only two things in this score that have any manual adjustments of any kind are the elements in the title frame and the last "repeat and fade", which is set to right aligned.

The bottom line is - use style settings and the Inspector well, and you will find autoplace helps you more than you could possibly have imagined.

So, I spent about 20mins on your score using 3.0.5, of which the first half was restoring automatic placement of all elements and resetting elements to their default positions. Also, just for certainty, reset the stretch on all measures.

To add text to the header, right-click it (or for Mac Ctrl-click) and select Add → Text. The "Ac. Guitar, Capo 7 and Slow cha-cha" are all part of the same text element, the separate styling is done by highlighting the relevant part and using the styling toolbar at the bottom of the window.
This allows us straight away to reduce the height of the title frame to 6sp.

Next was to reduce the insanely big/bold rehearsal marks into something readable. I manually styled and positioned the first one, then pressed the "set as style" buttons in the inspector to have it propagate to all of them.
Next I added line breaks before nearly all rehearsal marks, allowing them to flow into the page margin, just as in your current score.

Similarly, reset all chord symbols to the style I gave the first one.
Staff texts that should be above the staff were changed to placement "above" and once again a default position was set using the first of those markings. Similarly, a default position is set for those that have placement "below" as well.

Replaced the first coda sign with the actual "to coda" instruction (so playback now understands it too), then changed it's text to read as the coda symbol. Changed the size of it and the Segno to be 24pt instead of 25pt, which hardly makes a visible difference, but is a more standard fontsize; so likely the printout of them will be sharper.

This is the point where you realize you simply have too much music to fit on a single page. Looking at the scaling of the score, you're currently using a whopping 1,85mm for an sp, which is quite large. Reducing this to a still very acceptable 1,6mm makes everything almost fit on a single page, just one system too much.
Going into the style settings and reduce the minimum system distance from 8,5sp to 8sp makes everything fit on the page.

Also added a frame to more clearly separate the coda from the main piece.
Added line breaks to result in a more balanced distribution of measures across the systems. Reduced stretch once for the measures of the Chorus&Break part to make them fit on a single line.

Disabled automatic placement for the D.C. al coda to allow it to reside below the staff.

Prevent measures from flowing onto the next system after line breaks are locked in, by selecting the full score and reducing stretch once.

So, if I didn't had to undo all your manual fiddling, that sums up to just about 10 mins of styling work (without knowing the piece up front; imagine if I actually knew how it should look!) and disabling automatic placement on just a single element.

See attached for the resulting score and a comparison view.

[EDIT] post crossed with Marc's from above, but we're basically saying the same thing. Use styles to set default positions as you want them, and you'll find automatic placement working for you, rather than against you in most situations.