I think it would definitely be easier and safer to go from a 40ft board or up. I think a 34+ board could probably work unless the third phase is just awful and the first two are really big.
As far as the shin pain, there are preventative measures you can take early on in a season and also some things you can do once you’ve started to get them but it’s a lot harder to get rid of shin splints than it is to just try and prevent them from happening. I would recommend doing more grass type running and not wearing spikes as much earlier on. Also eliminating some of the pounding on non sprint days could be good. Mixing things up with maybe a swim/bike/row workout instead of running can always be a good idea. If you already have shin pain then ice bath or ice massage stuff can help. Also I’ve seen some athletic trainers have athletes do different exercises to strengthen the shins such as picking up things with the toes and playing them in a cup on the ground and such, not sure how effective that stuff is but who knows.
As far as getting recruited for D1 and D2 schools, it’s never too late to get recruited. I mean you won’t get as much attention and have maybe as many opportunities the longer you wait but if your numbers are good enough and the team could use then there is plenty of time.