Short answer, you're locked in.

You can only choose one archetype per class, and there are no current rules that allow you to pick-and-choose features from different archetypes.

If you want to diversify your character's build, you can look into multiclassing. This lets your character pick up levels and features from different classes.

Note: You cannot multiclass into the same class to get additional archetypes (relevant Sage Advice).

I would recommend taking a read of this article: Modifying Classes. Whilst it doesn't discuss exactly what you're after, it might help. Specifically, the part on Creating New Class Options.

One way to get what I think you're after is to create a new Roguish archetype. It's a bit of a cop out, but with your DMs blessing it would definitely work. You'd have to come up with a very strong reason why your class would be meshing two existing archetypes (Thief & Assassin) though; and from a balance perspective you could come across certain situations where your character has too many tools at its disposal compared to other PCs which stuck to a single class option. Once again, discuss with your DM.

Back to the article, the key takeaway is this:

Take a look at the class’s existing options and see what they provide, and then use those as examples or building blocks for the features that your class option will provide. It’s perfectly fine for two class options in the same class to share some mechanics, and it’s also appropriate to examine other classes for mechanics you can draw upon for inspiration.

If you get an okay from the DM to create your Thiefsassin, you could utilise the Thief features for 3rd and 9th level, and the Assassin features at 13th and 19th level.

If their maximum hit points is zero then healing magic won't do anything. Healing magic allows a character to "regain" or "restore" hit points, which is always up to your maximum.

When a creature receives healing of any kind, hit points regained are added to its current hit points. A creature's hit points can't exceed its hit point maximum, so any hit points regained in excess of this number are lost.

Basic Rules, Combat chapter.

This disallows cure wounds and healing word and their mass versions, prayer of healing, goodberry (assuming you can feed a berry to an unconscious person) and pretty much all healing potions.

One spell that will help is aid - as long as you keep casting it over and over again.

Your spell bolsters your allies with toughness and resolve. Choose up to three creatures within range. Each target's hit point maximum and current hit points increase by 5 for the duration.

Please note that many effects that reduce maximum hit points also specify really bad things for when the character reaches 0 (for example, the strength drain attack of a shadow). Keeping someone conscious isn't much of an issue when they've turned into an undead monster and are trying to eat your face.