When you are cleaning the old mortar, paint etc off the faces with acid you are also removing the years of weathering that you get with a reclaimed brick.
I wouldn't recommend anything like Thompsons as you will seal them stop the brick from breathing and may cause more problems in the future with frost damage due to sealing in moisture,
If you want a darker finish I would look at tinting them darker with a brick tint from a company like Bebbingtons
https://bebbingtonbrick.co.uk/ that way the integrity and breathability of the brick is retained.
Feel free to drop me a DM if you need any more advice.