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Sergio Ballestero

@sergioballestero

Senior Full-Stack Engineer (React, Next.js, NestJS, PostgreSQL) with a design background. 18+ years, building SaaS for US teams.

Paraguay

Message

What I'm looking for

Remote senior full-stack role with a US product team, owning features end to end in React/Next.js and NestJS or FastAPI on PostgreSQL. I design in Figma and build what I design, so I fit product-minded teams that ship often and value clear async communication. Based in Paraguay (UTC-3), good US overlap; open to long-term contract or full-time.

At Welcomy.io, I took a multi-tenant queue-management SaaS from schema to production as its sole engineer. I built its ticketing, priority handling, kiosk displays, and visitor analytics, along with AI Insights and Forecasting that can route between Claude, Gemini, and a locally hosted model.

As an independent Full-Stack Engineer, I’ve worked with US and regional product teams across proptech, VoIP, and workflow automation. At Hilagro (TransAgro), I designed and built a supplier and accounts-payable portal that automates invoice-to-purchase-order reconciliation; at BrightReps FlowEQ, I built a workflow step editor and analytics.

Earlier, I built the university intranet and student and teacher mobile apps for Universidad del Pacífico, and led development of a GPS fleet-tracking platform at Global Tracking Paraguay. Formal graphic design training informs how I work: I design interfaces in Figma and then build them.

Experience

Work history, roles, and key accomplishments

Current

Founder & Principal Engineer

Current

Welcomy.io

Feb 2026 - Present (8 months)

Multi-tenant queue-management SaaS — ticketing, priority handling, kiosk displays and visitor analytics. Sole engineer across backend, frontend and infrastructure; deployed to production and recruiting its first pilot users.

NestJS TypeORM PostgreSQL JWT OAuth RBAC pg boss Next.js Astro Docker Traefik Cloudflare Claude APIs Gemini Ollama Hetzner Coolify

Current

Independent Full-Stack Engineer

Current

Self-Employed

Aug 2024 - Present (2 years 2 months)

Long-running retainers with US product teams. ForFounder: FastAPI/SQLAlchemy auth and security, Stripe and HubSpot, Next.js admin. FlowEQ: React/TypeScript step editor with React Flow, AWS Lambda and Step Functions. ADIGI PlaceAnalyzer: design system as an npm package (MUI, Storybook) and a Next.js PWA. Hilagro: Next.js/NestJS/PostgreSQL supplier portal with ERP integration.

Senior Frontend Developer

In All Media

Jul 2023 - Aug 2024 (1 year 1 month)

Senior frontend developer embedded with Ooma (US VoIP). Shipped features in React, TypeScript and Redux across the HIPAA compliance module, signup flows, messaging, dashboard, integrations and account preferences; integrated REST APIs with backend teams and mentored junior developers through code reviews and pairing.

Full-Stack Engineer

Universidad del Pacífico

Aug 2021 - Nov 2023 (2 years 3 months)

Designed and built the university intranet and its student and teacher mobile apps — rolled out to the entire active student and faculty body — across the web frontend, the API and React Native.

React React Native Firebase Bancard Moodle CRAI PDF Excel NodeJS

Full-Stack Engineer

Jacob Tyler

Mar 2023 - Sep 2023 (6 months)

Built WordPress sites and PHP API integrations for US consumer brands through the agency: Upper Deck (membership API sync), Jetty Extracts (ActiveCampaign and a custom age-gate plugin for compliance), BayMedica and BrocShot landing pages, plus a Next.js build for Centro Hub.

Full-Stack Engineer

number8

Jun 2022 - Mar 2023 (9 months)

Delivered applications for US education and e-commerce clients in React, Next.js, Angular and React Native, including a scholarship and grant management platform built from Figma wireframes. Integrated Okta Workflows and worked test-first with Enzyme under team code review.

Senior Software Engineer

Rockstar Coders

Jan 2021 - Jun 2022 (1 year 5 months)

Built and maintained wedding-planning platforms for US companies inside a multi-project monorepo, delivering responsive Angular and React components from design wireframes and the APIs behind them. Worked test-first with Enzyme on a feature-branch workflow with code review.

Full-Stack Developer

Global Tracking Paraguay

Apr 2016 - Jan 2021 (4 years 9 months)

Led development of a GPS fleet-tracking platform used over 1,000,000 times, taking messages from more than 2,000 vehicles every 15 seconds, including the UDP server that processed hardware events and SMS commands.

React TypeScript WordPress PHP Laravel NodeJS Docker Gitlab CI

Design Manager

Puntopy

Jun 2007 - Apr 2016 (8 years 10 months)

Led design and web development for digital projects and campaigns for brands including Unilever, Miller Genuine Draft and Tigo, from concept to execution: websites, apps and touch-panel kiosks for shopping malls, delivered on GitLab with a branch-per-feature workflow under Scrum.

Wireless Engineer

Tigo Paraguay

Apr 2006 - Feb 2007 (10 months)

Supported Tigo enterprise clients, installing and maintaining wireless infrastructure and troubleshooting Wi-Fi installations on site, and assisted backend operations with PHP and Oracle.