I'm new to reloading and as such beginning to gather things together. I'm thinking of develvoping loads for my 243/ 100 gr. SBT, 270/ 130gr. SBT and 30-06/ 150gr. SBT using either RE-19 or H4350. Any help or advice will deeply appreciated.

Well for the 270 and 06, 4350 is the goto powder. I like IMR myself, but either will work. For the 243, varget might be a better choice.

been using 4350 for 25 years now and never had a problem

IMR 4350 is pretty much my "go to" powder.

ditto. I like 4350.

IMR 4350 gets my vote

Wow by now its redundant IMR 4350 for the 270 and 06 the 243’s I had were picky and I ended up using a now discounted powder H450 that was some 20 years back

It is best to use a powder that pretty much fills up the case. I believe the powders you listed, 4350 and RL 19, are both going to do that in the 243 case. They should both be quite good for your application. RL 19 and 4350 are also both quite good in the 270, and the 30/'06 as well. These two are so close together, one is not significantly better than the other.