I had the cc2531 and was having similar issues. I have the first one in the link provided by Francisp above- the Electrolama zzh, which was shipped from the UK and came in a couple weeks to the US- It’s worked great since. Probably any based on the upgraded CC2652R chip will work better.
The cc2531 is noted as “not recommended” in that list. Maybe it would be ok for a couple devices, but definitely not 20. It just doesn’t have enough processing power.
Update: further down the list on the link it says “up to 20 devices” for the cc2531, so you are at the very top end then.
Also, watch out for the slaesh one, a lot of complaints about ordering it and it never shows up