I agree with your concept of reducing trimming but roundup an most herbicides are not recomended under trees. It can kill them.
I would recomend concrete edgers that can be mowed up to with no need to trim around. An L shaped design that the mower deck can ride over. Sure thats a while lot of work and expense once but better than all that trimming.
Also agree with zero turn with a 5ft deck will cut down that mowing time 40-60%. Another thing to consider to reduce mowing time is to reduce yard space with low maintenance flower/rock gardens with that same trimmer free edging. Use good thick plastic and river rocks the planters with some low maintenance spillover vines and boom, landscape thats no weeding or mowing or trimming.
Good luck.
I use a 61" deck, and this is how I avoid trimming too. I mow somewhere around 7 acres weekly, consisting of a large lawn, trails, camp site, etc. Well, to be honest my son usually mows at the same time I mow, and we're done with it all in less than 1 1/2 hours. I have hundreds of trees, many mulched beds, and obstacles galore, and our landscaping around the house and barn is curved mulched beds for appearance. But I trim nothing. Everything can be mowed by overhanging the deck or around sprayed trees, and I don't have to trim a thing. I enjoy mowing, and relax while I mow and listen to the radio or music.61" mower deck is about right for 4 acres.
Use mulch and/or roundup around your trees, fences, etc and never worry about running a string trimmer again. I havent ran one in probably 8 years. If my 61" mower cant mow it....I spray it
Agree with ash and maple saplings. I have killed them with multiple roundup sprays but i did serious damage spraying near a blue spruce once. Not all trees are equally resistant to roundup and there are much more potent herbicides out there that can easily kill trees by accidentI spray around all my trees, from mature 30" trees to 1' high trees that are no bigger around than a pencil. Probably spray 4 or 5 times a year. Never killed any of them.
Roundup has to get on foliage (something green). Or it simply wont work. So as long as you arent spraying the leaves, the chances of killing a tree are nil.
In fact, even if the leaves are soaked in roundup, the chances of permanently killing the tree are slim. I have tried to kill ash and maple trees that start growing in flower beds and the garden with roundup. Leaves wilt, turn brown, then come back with a vengeance.