Sam Muir plays both Classical Guitar and Ukulele.
The most efficient and effective pathway in my opinion is to take a break from ukulele for about 12-18 months and sign up for classical guitar lessons. Then come back to the ukulele. Learning the classical guitar left hand techniques is the key to starting to be able to play as well as Sam does. If you can apply yourself for a whole year, under instruction from a good teacher, you will improve a lot. But it is hard to find the cost (30 - 50 lessons at $20 each = $600 - $1000) and the time for lessons (40 minute lesson each week and 30 minutes a day) , and probably there are going to be some lessons on music theory etc. to get through.
If the lessons are too hard, consider buying a low cost 3/4 or 1/2 sized guitar. Two examples are the Yamaha CS40 (3/4 size) and the Yamaha GL1 (1/2 size). They will probably cost around $150. Maybe there is a used one in a local shop. I am not recommending to buy them to replace your ukuleles. They would be like gym equipment to help you develop your ukulele skill set, so sound and intonation are not a priority. Once you have the six stringed instrument, you buy a book of studies or arpeggios and work on them to get your left hand in shape to play your ukulele. The extra stretching will grow your left hand. You do need to start slowly and keep working at it. Over 18 months, you should be able to get a lot more control over your left hand.
If you want to stick to ukulele, then look at something that might be enjoyable and not repetitive or boring. Instead of learning strums to accompany your favourite tunes, learn arpeggio patterns. You can work them out your self. First you write out the chord progression. Then you look up the notes that are in each chord. Then you find those notes on your ukulele fretboard and bar by bar, write out the arpeggio pattern. Some trial and error and noodling is required to refine the patterns. Once you are happy with the pattern write it out as TAB, with the lyrics, and start to play the tune. The first 10(?)times it may be hard to sing along, but with persistence you will get it working, then you will be enjoying singing and playing and your left hand will get a workout.