Configuration of my Meridian system. G68 after service, adding HD621. The old laptop with serial has half-dead RAM (around 100Mb RAM is *sometimes* working...)
HD621, G98DH, G68ADV, Bc Acoustiqe ACT A3, 5200C DSP33, NuForce Reference 9 monoblocks, 2x SB3's, Velodyne DD-15 & DD-12 Subwoofers and a Meridian sub.
I've used StarTech, but not on Sooloos. I was pushing its limits and worked well. The com port retention is a boon for windows users although that an FTDI thing on some of their chipsets rather than specific to StarTech.
I don't know if that has null modem functionality built in, but if it has, it can have unforeseen issues when moving to SpeakerLink as many of those have null modem built in so you still need null modem and in a set of genders that are not popular so may need to be purchased once exactly what is needed is known. In addition, I'm not sure what gender the StarTech DB9 connector is - you may still need a straight cable (rather than null modem crossover) to do what you want.
Be warned, many counterfeit FTDI chipsets in the supply chain and these are sometimes targeted by FTDI drivers and are 'disabled'. Unfortunately, even some of the big names got hit by this so a known brand is no guarantee of chip providence but I'm sure the StarTech will be good.
Meridian owner since 1992 Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).