Replacing our current tracksuit supplier - 15,000 garments/year, quality issues

We have sourced our fleece tracksuit line from the same supplier for a while now and the relationship has run its course: GSM has been drifting lot to lot, the heat-transfer prints crack after a handful of washes, and lead times have gotten unpredictable. We cannot keep absorbing customer returns over it. We need a manufacturer who can take over the programme: two-piece hoodie and jogger sets in brushed polyester/cotton fleece, 380-450 GSM, sizes S through XXL with a plus-size grade, drawstring hood and elastic-cuff joggers. We print our own logo via heat transfer and embroider on request. Volume is around 15,000 garments across the full year, released in regular batches rather than one lot - we need a partner who can sustain that cadence without the quality sliding the way our current one has. Before switching fully we would want a trial batch to validate GSM consistency, print durability and stitching, plus documentation: azo-free dyes and an OEKO-TEX Standard 100 certificate. Master cartons of 20, barcoded to our warehouse system. Delivery DDP Atlanta GA. In your proposal, walk us through your quality control process, whether you can run a trial batch before we commit to full volume, pricing at trial and at full volume, and how you handle a supplier switch mid-programme without disrupting our releases.