I always use a Shetland for first timers if I possibly can. Easy to lamb, very active lambs jump up and find the milk bar straight away. Mum learns her job with practically no stress. From those mothers you’ll have lambs which will finish well enough by the end of the summer / early autumn no bother.
Then next time, they all know their jobs and their ligaments will be nice and relaxed, so from that cross you could use pretty much any ram you chose.
You can get polled Shetlands but it might take a bit of searching. However, only the male lambs would have horns anyway, and if you castrate as promptly as you are able, the horns will be pretty small.
If you don’t fancy Shetland, my next pick would be a Charolais, for many of the same reasons (also easy to lamb, very active lambs jump up and find the milk bar straight away. Mum learns her job with practically no stress. Just need to be aware that the lambs will have very light fleece at birth, so in bad weather either have the mums inside and or jacket the lambs.). Charolais cross from those mothers should give top quality and top performance fat lambs.
If you use Charolais and are tempted to keep any of the ewe lambs on, be aware that Charolais will start to cycle much earlier than many other breeds
Other than that, Charolais x make fab ewes.