Glad you like! The main thing for solo fights is to give more action economy to the solo enemy.
Some things I did recently with a fight against a beefed up Warforged Titan (called an Arcane Titan)
It had absorb elements, heat metal on it's weapons, and it exploded when destroyed
It had an arcane energy shield that it could regenerate by spending charges, basically like the artillerist's THP cannon.
It also absorbed part of any spell with a level that was used on it, gaining charges as if it were a magic item. (in story, the cult that made it had mixed creature magic and magic item magic. Next version they face will also have place magic, and thus lair actions!)
It had a bonus action arcane cannon, as the artificer's eldritch cannon, and two emplacements for snipers to sit and have half cover.
Each of it's major systems was a creature with it's own hit points and AC and 1 action per round independent of it's turn. These systems were each arm, the legs and lower torso, the upper torso and head, and the cannon up top.