When you encounter a fastener that your current tools will not allow you to install properly, the thing to do is find or make the correct tool that will work.
At no point is it a good idea or acceptable to just leave the bolt loose.
If this was my truck and my problem, I would disconnect the negative battery cable, remove the starter and bench test it, and inspect the connections at the battery, and starter.
If the starter test good and all of your connections are good reinstall the starter but make sure to do whatever is necessary to make sure it is installed correctly or pay someone competent to do the work for you.
When you added water to the battery did you use distilled water or tap water?
How much water did you put in? hopefully just enough to cover the cells.
If you are unsure if the battery is good or not have it load tested.
When dealing with hard to reach fasteners and a limited budget, you have to be creative.
I used to have in my tool box metric and SAE T handle flex head wrenches from Snap On I know they are expensive.
https://store.snapon.com/T-Handle-in...h-P633434.aspx
Yet these long handles can be bent to nearly any shape allowing them to work in places no other tool could.
Keep in mind if you bend them too much the chrome will peel off in spots.
Once your done just straighten out the tool for the next job.
I am not sure if this T handle ratchet wrench idea will help or not but ideas like this is what you will need to get that starter bolt tight without pulling your hair out.
Once the starter is installed properly and has been bench tested and the connections verified good if your still having a issue let us know.