C4 is more than adequate.
Draw the boxes and lines and make sure that people understand what they mean. Describe the system from the perspective of the reader. Just like a real architect will have different drawings/plans/elevations for their customer, the planning authorities, and the builder.
The architecture diagrams are meant to communicate the design, not comply with some over-worked standard.
UML was a clusterfuck that evolved from the trifecta of late 90s OOP (inheritance not composition), design patterns that mostly provided fill-ins for what was missing from Java as a language, the ridiculous concepts of generating code from diagrams that could be regenerated from code, which never, ever, worked, in the same way that ORMs have an impedance mismatch between OO and relational logic.
It was yet another silver bullet, the late 70s/early 80s had "structured programming", the late 80s/early 90s had CASE, the 90s had all the stupid diagramming tools where people argued about the shape of the bubbles and what arrows to put on the lines.
There was also the Capability Maturity Model, where everyone was trying to get to "Level 5" which was only useful if you were doing exactly the same software over and over again, along with the "6 Sigma" and "Black Belt" nonsense.
The 2000s had the "iterative Rational Unified Process" (an excuse to sell expensive tools from IBM), along with CORBA et al.
The last decade has suffered from the Agile-with-a-capital-A and especially "Scaled Agile" which is just an excuse for project managers to again treat programmers as fungible, while losing all of the affordances of actual project management, like GANTT, critical path, S curves, earned value etc.
Sprints are nonsense, so is "t shirt" sizing and velocity and burn down charts, and retrospectives and scrum "masters".
There are a couple of useful things:
* Domain Driven Design, using Names That Make Sense To The Customer
* Entity Relationship Diagrams, where the Entities are the same Names as the DDD
* State Diagrams for those Entities, describing the events that will cause them to change state
* Event definitions that match the State Diagram transitions, where externally generated events end up being your external API.
Bah, I've been in this industry for almost 40 years and its the same shit over and over, just with different names, and different consultants selling expensive courses.