You maintain concentration, regardless of the form. Your ability to concentrate on a spell does not depend on any mental statistic. You can also continue to concentrate on a spell, even if you cannot cast spells. For example, Silence, Druid Wildshape, and the Antimagic Field spell all restrict spellcasting, but not do not restrict your ability to concentrate. If, hypothetically, your concentration were tied to your ability to cast spells, then a cleric who casts Antimagic Field would immediately lose concentration, and the spell would be incapable of having an effect. From this we can safely conclude that these two mechanics are not intrinsically tied to one another.

Page 203 lists the requirements for concentrating on a spell. You lose concentration if you:

• Cast another concentration spell
• Take damage
• Are incapacitated or killed
• Are subject to environmental phenomena (DM's discretion)

Being polymorphed does not fall under any of these categories. A character can therefore maintain concentration while polymorphed.

Yes you can true polymorph into an ancient brass dragon

As you say, the spell and rules are clear. An ancient brass dragon is a creature, it is not a shapechanger (and may not matter if they were), and it is of the appropriate CR for your level.

Thus, there is no reason why you would not be able to use true polymorph in this way.

Ancient Brass Dragons are not shapechangers

Ancient Brass Dragons are not considered shapechangers. A creature is a shapechanger if they have the monster subtype (Shapechanger). Ancient Brass Dragon's creature type is dragon.

The fact that it is able to change shape (a special ability) is irrelevant to being able to transform into that shape. The dragon does not have the shapechanger subtype, thus it won't be potentially conflicted with the shapechanger subtype restriction.

You can use all the abilities of your True Polymorph form

The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form.

Jeremy Crawford says:

When a game effect, like Wild Shape, says you use a creature's statistics, you use its stat block (see "Statistics," MM, 6).

According to that same section "Special Traits, Actions, and Reactions" are part of a monster's stat block.

Ability to change shape back to your old form is unclear.

It is worth noting that you may not be able to use change shape to change back into your old form: Can you use a Dragon's Change Shape Ability to Turn Back Into Yourself?