As you know you really shouldn't wean until they are ready and never before 17 weeks. Being ready is when they are holding their head up, sitting up straight with minimal support (ie not being slumped in a highchair but you can use the shoulder straps to support them a bit if they aren't sitting completely alone), have lost the tongue thrust reflex (if they still have it their tongues will push food out of the mouth and they won't be able to swallow it all only little bits), able to pick small chunks of food up, put them in the mouth, chew and swallow it properly. When they are doing this then you can go straight onto finger foods and skip the puree stage. It generally happens some time between 17 and 26 weeks but usually closer to 26 weeks than 17 weeks - it can happen at 17 weeks but it's rare. it's so much easier cos you can just give them what you are eating without faffing about with a blender and worrying about avoiding lots of foods.
Being ready for weaning is not when they have doubled their birth weight, their weight gain is slowing, they are looking at food, feeding more frequently and waking up in the night more. The last two are most commonly associated with a growth spurt which can last up to 8 weeks - a fact unknown by many. Growth spurts usually only last a few weeks but they can last much longer than this.
Weaning before they are showing the signs i mentioned in the first paragraph should only be advised by a paediatric nutritionist. they are the people who are experts in this area - more so than a hv/gp/general paed/dietician. in very rare cases a paed nutritionist might advise early weaning for example if a baby is losing weight or if they have crossed the thrive line. to my knowledge they would not advise it just because they are not gaining much and have fallen off the growth charts (i got told that as long as there is a gain that is good), they are taking too much milk, are feeding every hour, have constipation or reflux. i had all this with ds and i was explicitly told not to wean early as it could make things worse.
if a paed nutritionist did advise weaning before 26 weeks they would say to steer clear of anything except for fruit and veg. this is because things like nuts, fish (both normal fish and shellfish), meat, gluten, rice and dairy are frequently things people are intolerant to. even with just sticking to fruit and veg weaning before 26 weeks is still quite difficult because fruits and veg that you should avoid before 26 weeks are apples, pears, plums, cherries, nectarines, peaches, mangoes, melons, bananas, kiwis, raspberries, grapes, strawberries, citrus fruits, tomatoes, mushrooms, peas and peppers because they are common allergens aswell. the most common fruits to be allergic to usually have seeds or stones and the vegetables are usually beans or legumes. doesn't leave you with much to try does it!! even the gluten free, sugar free rusks are sweetened with apple or grape juice juice. Basically if you do end up weaning before 26 weeks make sure you avoid the things i mentioned above otherwise you don't need jars or rice you can just puree your own fruit and veg.
HTH