Is not going to be viewed from a great distance
- Define your viewing distance.
Reading a text on a post-it, or reading the same size text from a poster does not depend on the size of the paper, it depends on the viewing distance and the size of the text.
There are some other factors like the font design, but let's think that you are using a simple sans serif font like arial.
I don't want any ill surprise during the printing process
- Define what is the "printing process".
The only way to avoid any "surprise" is actually using that printing process before.
Go and ask for a print sample similar to what you need to print. In fact, send a small size sample, for example, a small strip of 30 cm linear (they print a roll, so your sample should be the same size of the roll x the length of the file.
But probably the other surprises will come from your design process, for example, if you are not working with vector files.
A1 is a beast, so I think it could be even a quite small font size (like 12-11?)
It seems that you are thinking of some kind of re escalating. Am I right?
If you work at a scale, you need to think all at scale. If you want a final size of A1, you could make a design on an A3 artboard which will be at 50% scale.
Working with an A4 artboard will be confusing because the scale is 35.35%
In short. Define the viewing distance, print a sample text on your home printer and paste it on the wall. Take notes.
Define your scale. 50% is a good, easy to work scale.
Send the file in vectors. Print a small sample using the same process as the final files.
Take a look at this similar questions.
What line width is appropriate for a scientific A0 poster?
Artwork size - downsampling