Frist of all, if you are playing an old save on a new patch, migrate immediately as some of the changes in the patch will not be in effect on old saves untill you migrate. Such as Straw.
You do not need farming skills to till, plant or harvest.
If you adjust priorities in Farming groups to always be max, then lower all others to default, or 1 tick below max, the farmers group will always be filled when tilling, planting or harvest season starts.
Remember that when the farms do not need any work, your farmer group members will join other groups that has tasks to do and a spot available. This is working as intended so empty farmer groups in for example December is correct. If there is no available spot for them anywhere else they will stay in the farmer groups but go do communal tasks and chores.
You can also increase the group task priorities in the policies to make sure groups have priority over the communal tasks by moving slider to the left.
If you do not have any farmer groups, all farmland is then by default; communal tasks, meaning it will be tended to by tribe.
So, if you wish the tribe to focus more on the tilling, planting and harvesting, you can during these seasons, adjust your communal task policy to favour communal tasks above group tasks, by moving the slider to the right. I would go max.
It sounds like your tribe have bitten over more than they can chew of work-force tasks versus income.
There is a balance that needs to be met in order for things to go smooth, and this is smt we as players must adjust for every new tribe and locality as they are all different. Also as the years go by.
If you cannot keep tribe afloat at population 67, no mats and not enough food after just 5 years with farming, then smt has gone very awry with your tribe balance.
I first of all recommend labour age to be as max as your tribe leader allows you.
Do not mess too much with workhours outside tilling, planting and harvest season; as in , change it to 1 notch to the right from default at those seasons, then lower it again before anyones health is suffering.
I also recommed that you take ādrasticā measures to correct the lack of balance by adjusting all groups.
As an emergenct solution to get tribe quickly back on the feet try this:
1: pending season: make 1 group , max vacancies and prioritiy that focuses on 1 foodtype, if autmn pick nuts, if summer pick rosehips.
For spring and winter max out your fishermen, and increase hunters to 8, if you have animals left in your locality that is, otherwise, create a 2nd fisherman group with 8 people in it
2: make 4 more gather groups:
1 group for straw only, 10 people
1 group for reed only 10 people
1 group for fibre, 2 people
1 group for sticks 2 people
Put all these group max priority and place all others 1 notch lower.
Let them massharvest till the food is in large amounts, at least 6 sleeps for entire tribe.
Nothing else is important for a while so leave other groups as they are even if empty.
Pause everything that needs repairs and do 1 hut at the time as enough mats come in, this can take some time if your entire building amount is in disrepair.
Dismantle any huts that are above the count of heads in tribe as you are effectively throwing mats out of the window. There is no need to make more huts than your actual population count.
This might be down to logistics as well, as the further away you have to send your workforce for same amount of income as 5 years ago; the more workforce is needed to achieve same amounts, due to loss of workhours to walking. Hence as time goes by, this should be adjusted bit by bit as you see income is too low for the workforce.
You should however with right balance, never have too little food in your tribe.
Food is always first, always always, so, let people go without huts for a while as you are working on the right balance, then you can build huts, 1 by 1, and also adjust balance as you see things are starting to go awry. If things keep going wrong, stop what youre doing, and figure out where the bottleneck is, then adjust accordingly and keep going.
If you cannot get out of this pickle I can help you further by watching your stream on Discord and see how you are managing your tribe as you go along.
I can then guide you where I see it might be needed. I can also help you get your tribe back on its feet.
Just let me know 
