How can you develop a user-friendly software solution for clients with varying levels of technical expertise?

I think first crucial thing is understanding client business and understand what they wanna achieve or what problem they wanna solve. Based on that we can prepare how end-user can get best of product/project. This may required to review of current market leaders to understand user behaviour and how we can define do's and don'ts.

Understanding the problem you are going to solve is the key. You need to break up the requirements into manageable pieces. In addition, it is also important to educate customers about how the solution works and how it impacts their processes so that the new solution integrates smoothly into their routine.

I agree but you also need to understand what real business value the solution will generate for the client. Goals and pain points need to translated in business value.

The system should do steps synchronously as less as possible to reduce the amount of time spent by the user on the app. Once the input is completed, use event based system to regularly notify of the progress being made and if the customer needs to perform any actions. If possible allow less technical user to trial the application allowing them to cancel the last step to get the "feel" of the service being offered. This can be simple as sending notification at every offline step. Ensure the system is fast and responsive. Nobody likes to wait for 10 seconds to get a error screen.

After clearing scope of project, choosing technology may be sometime hard as well. From starting with project documentation, design wire frame, code versioning, Unit test reports, Artifacts, CI/CD pipelines, hosting, security testing & auditing and a lot others needs to be identified with right tools.

You need to identify technical capabilities first and then mapping the correct technologies and tools for your company. Technologies must fit your company IT culture and knowledge, experience is a key factor for success.

Keep the Form elements very simple. Avoid text and replace them with choice ( drop down ). Too many text and options which has effect on final purchase should be avoided. Also ensure that descriptive text are available for less able people. Show icons and pics of what is needed upfront to ensure the submission process is easy to accomplish.

Regarding user interface one word: "Simple is better". Make it clean, neat & accesiable for every user & devices. Define typography , color schema and follow best practices of SEO, Performance etc based on business needs.

Assume your users know nothing about your application when designing it. It's easy to get entrenched into a subject area that domain relevant knowledge is overlooked, but remember new employees, new users, and users who are familiar with competitors will have to use your product. Additionally, make sure that you're designing for the accessibility needs that your user base may need. While you can control what internal group of employees needs to some extent, public apps should have accessibility testing. Also, a personal gripe: low-contrast fonts decreases readability. For about five years there in the mid-2010s, it became popular to put gray text on slightly lighter grey backgrounds. I stare at a screen all day. It is not readable.

Always have beta testers from outside the company / department to use the software and take feedback constructively. As part of the release do not make dramatic / too many changes to UI. People who used the application before will feel unsatisfied if too many changes are brought in one go. Think small changes with big impact

YES, but define the priority of features to make sure every deployments are fully functional with priority of features. I used to define unit testing with priority like P1, P2, P3. Where P1 are business critical & blockers and has to pass all test levels. P2s are more like High but can be fixed in 24hr period & P3s can be sorted in next release. Unit testing, Integration test, load test, performance test all needs to categories to get best of testing cycle.

Create help videos and host webinars for people who are going to use your application. Option to share the screen or talk to a customer support staff should be available at all time. If the application is having known issues then provide update upfront so that they dont waste their time to face error in the middle or at the end.

"Awareness & Training": Upon completion of project, Needs to provide training to client and supporting stakeholders. Training document may contain screenshot based document, Videos, FAQs. Frequently scheduling offline or online seminar is one of the best way to do this exercise.

Communication is key for your customers. Keeping them updated on project status, service tickets, and other aspects of their IT infrastructure keeps you ahead of the game.